diff --git a/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart b/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart
index 0885714ecc0e..ec6d37e01d82 100644
--- a/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/class_hierarchy_builder.dart
@@ -606,9 +606,17 @@ class ClassHierarchyBuilder implements ClassHierarchyBase {
   }
 
   Member getDispatchTargetKernel(Class cls, Name name, bool isSetter) {
-    return getNodeFromClass(cls)
-        .getDispatchTarget(name, isSetter)
-        ?.getMember(this);
+    ClassMember classMember =
+        getNodeFromClass(cls).getDispatchTarget(name, isSetter);
+    Member member = classMember?.getMember(this);
+    if (member != null && member.isAbstract) {
+      if (cls.superclass != null) {
+        return getDispatchTargetKernel(cls.superclass, name, isSetter);
+      } else {
+        return null;
+      }
+    }
+    return member;
   }
 
   Member getCombinedMemberSignatureKernel(Class cls, Name name, bool isSetter,
diff --git a/pkg/front_end/lib/src/fasta/kernel/forwarding_node.dart b/pkg/front_end/lib/src/fasta/kernel/forwarding_node.dart
index 53f6b2d6597d..0b4a1660c6a3 100644
--- a/pkg/front_end/lib/src/fasta/kernel/forwarding_node.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/forwarding_node.dart
@@ -162,6 +162,12 @@ class ForwardingNode {
     Arguments arguments = new Arguments(positionalArguments,
         types: typeArguments, named: namedArguments);
     Expression superCall;
+    assert(superTarget != null,
+        "No super target found for '${name}' in ${enclosingClass}.");
+    assert(
+        !superTarget.isAbstract,
+        "Abstract super target $superTarget found for '${name}' in "
+        "${enclosingClass}.");
     switch (kind) {
       case ProcedureKind.Method:
       case ProcedureKind.Operator:
diff --git a/pkg/front_end/lib/src/fasta/kernel/member_covariance.dart b/pkg/front_end/lib/src/fasta/kernel/member_covariance.dart
index 956f0fdfdc80..5d1d77fd0eef 100644
--- a/pkg/front_end/lib/src/fasta/kernel/member_covariance.dart
+++ b/pkg/front_end/lib/src/fasta/kernel/member_covariance.dart
@@ -61,14 +61,25 @@ class Covariance {
   /// If no type parameters are generic-covariant-impl, this is `null`.
   final List<bool> _typeParameters;
 
-  const Covariance.internal(
-      this._positionalParameters, this._namedParameters, this._typeParameters);
+  Covariance.internal(
+      this._positionalParameters, this._namedParameters, this._typeParameters) {
+    assert(_positionalParameters == null ||
+        _positionalParameters.any((element) => element != 0));
+    assert(_namedParameters == null ||
+        _namedParameters.values.isNotEmpty &&
+            _namedParameters.values.every((element) => element != 0));
+    assert(
+        _typeParameters == null || _typeParameters.any((element) => element));
+  }
 
   /// The empty covariance.
   ///
   /// This is used for all members that do not use any covariance, regardless
   /// of parameter structure.
-  const Covariance.empty() : this.internal(null, null, null);
+  const Covariance.empty()
+      : _positionalParameters = null,
+        _namedParameters = null,
+        _typeParameters = null;
 
   /// Computes the covariance for the setter aspect of [field].
   ///
@@ -287,7 +298,7 @@ class Covariance {
     int hash = 0;
     if (_positionalParameters != null) {
       for (int covariance in _positionalParameters) {
-        hash = hash * 13 + covariance.hashCode * 17;
+        hash += covariance.hashCode * 17;
       }
     }
     if (_namedParameters != null) {
@@ -297,7 +308,9 @@ class Covariance {
     }
     if (_typeParameters != null) {
       for (bool covariance in _typeParameters) {
-        hash = hash * 29 + covariance.hashCode * 31;
+        if (covariance) {
+          hash += covariance.hashCode * 31;
+        }
       }
     }
     return hash;
@@ -312,12 +325,10 @@ class Covariance {
             other._positionalParameters == null) {
           return false;
         }
-        if (_positionalParameters.length !=
-            other._positionalParameters.length) {
-          return false;
-        }
-        for (int i = 0; i < _positionalParameters.length; i++) {
-          if (_positionalParameters[i] != other._positionalParameters[i]) {
+        int positionalParameterCount = max(
+            _positionalParameters.length, other._positionalParameters.length);
+        for (int i = 0; i < positionalParameterCount; i++) {
+          if (getPositionalVariance(i) != other.getPositionalVariance(i)) {
             return false;
           }
         }
@@ -326,13 +337,12 @@ class Covariance {
         if (_namedParameters == null || other._namedParameters == null) {
           return false;
         }
-        if (_namedParameters.length != other._namedParameters.length) {
-          return false;
-        }
-        for (String name in _namedParameters.keys) {
-          int covariance = _namedParameters[name];
-          int otherCovariance = other._namedParameters[name];
-          if (covariance != otherCovariance) {
+        Set<String> names = {
+          ..._namedParameters.keys,
+          ...other._namedParameters.keys
+        };
+        for (String name in names) {
+          if (getNamedVariance(name) != other.getNamedVariance(name)) {
             return false;
           }
         }
@@ -341,11 +351,11 @@ class Covariance {
         if (_typeParameters == null || other._typeParameters == null) {
           return false;
         }
-        if (_typeParameters.length != other._typeParameters.length) {
-          return false;
-        }
-        for (int i = 0; i < _typeParameters.length; i++) {
-          if (_typeParameters[i] != other._typeParameters[i]) {
+        int typeParameterCount =
+            max(_typeParameters.length, other._typeParameters.length);
+        for (int i = 0; i < typeParameterCount; i++) {
+          if (isTypeParameterGenericCovariantImpl(i) !=
+              other.isTypeParameterGenericCovariantImpl(i)) {
             return false;
           }
         }
@@ -354,4 +364,71 @@ class Covariance {
     }
     return false;
   }
+
+  @override
+  String toString() {
+    StringBuffer sb = new StringBuffer();
+    if (isEmpty) {
+      sb.write('Covariance.empty()');
+    } else {
+      sb.write('Covariance(');
+      String comma = '';
+      if (_positionalParameters != null) {
+        for (int index = 0; index < _positionalParameters.length; index++) {
+          if (_positionalParameters[index] != 0) {
+            sb.write(comma);
+            sb.write('$index:');
+            switch (_positionalParameters[index]) {
+              case GenericCovariantImpl:
+                sb.write('GenericCovariantImpl');
+                break;
+              case Covariant:
+                sb.write('Covariant');
+                break;
+              default:
+                sb.write('GenericCovariantImpl+Covariant');
+                break;
+            }
+            comma = ',';
+          }
+        }
+      }
+      if (_namedParameters != null) {
+        for (String name in _namedParameters.keys) {
+          int covariance = _namedParameters[name];
+          if (covariance != 0) {
+            sb.write(comma);
+            sb.write('$name:');
+
+            switch (covariance) {
+              case GenericCovariantImpl:
+                sb.write('GenericCovariantImpl');
+                break;
+              case Covariant:
+                sb.write('Covariant');
+                break;
+              default:
+                sb.write('GenericCovariantImpl+Covariant');
+                break;
+            }
+            comma = ',';
+          }
+        }
+      }
+      if (_typeParameters != null) {
+        sb.write(comma);
+        sb.write('types:');
+        comma = '';
+        for (int index = 0; index < _typeParameters.length; index++) {
+          if (_typeParameters[index]) {
+            sb.write(comma);
+            sb.write('$index');
+            comma = ',';
+          }
+        }
+      }
+      sb.write(')');
+    }
+    return sb.toString();
+  }
 }
diff --git a/pkg/front_end/test/member_covariance_test.dart b/pkg/front_end/test/member_covariance_test.dart
new file mode 100644
index 000000000000..6f541df4906f
--- /dev/null
+++ b/pkg/front_end/test/member_covariance_test.dart
@@ -0,0 +1,214 @@
+// Copyright (c) 2020, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:expect/expect.dart';
+import 'package:front_end/src/fasta/kernel/kernel_ast_api.dart';
+import 'package:front_end/src/fasta/kernel/member_covariance.dart';
+
+main() {
+  void checkEquals(Covariance a, Covariance b) {
+    Expect.equals(a, b);
+    Expect.equals(a.hashCode, b.hashCode);
+  }
+
+  checkEquals(const Covariance.empty(), new Covariance.empty());
+
+  Expect.isTrue(const Covariance.empty().isEmpty);
+  Expect.isTrue(new Covariance.empty().isEmpty);
+  Expect.isTrue(new Covariance.internal(null, null, null).isEmpty);
+
+  checkEquals(
+      const Covariance.empty(), new Covariance.internal(null, null, null));
+
+  Expect.throws(() => new Covariance.internal([], null, null));
+  Expect.throws(() => new Covariance.internal([0], null, null));
+
+  checkEquals(
+      new Covariance.internal([Covariance.GenericCovariantImpl], null, null),
+      new Covariance.internal(
+          [Covariance.GenericCovariantImpl, 0], null, null));
+
+  checkEquals(new Covariance.internal([Covariance.Covariant], null, null),
+      new Covariance.internal([Covariance.Covariant, 0], null, null));
+
+  checkEquals(new Covariance.internal([0, Covariance.Covariant], null, null),
+      new Covariance.internal([0, Covariance.Covariant, 0], null, null));
+
+  Expect.throws(() => new Covariance.internal(null, {}, null));
+  Expect.throws(() => new Covariance.internal(null, {'a': 0}, null));
+
+  checkEquals(new Covariance.internal(null, {'a': Covariance.Covariant}, null),
+      new Covariance.internal(null, {'a': Covariance.Covariant}, null));
+
+  Expect.throws(() => new Covariance.internal(null, null, []));
+
+  Expect.throws(() => new Covariance.internal(null, null, [false]));
+
+  checkEquals(new Covariance.internal(null, null, [true]),
+      new Covariance.internal(null, null, [true, false]));
+
+  Covariance covariance = new Covariance.internal([
+    Covariance.Covariant,
+    Covariance.GenericCovariantImpl,
+    0,
+    Covariance.Covariant | Covariance.GenericCovariantImpl
+  ], {
+    'a': Covariance.Covariant,
+    'b': Covariance.GenericCovariantImpl,
+    'd': Covariance.Covariant | Covariance.GenericCovariantImpl
+  }, [
+    false,
+    true
+  ]);
+
+  Expect.equals(Covariance.Covariant, covariance.getPositionalVariance(0));
+  Expect.equals(
+      Covariance.GenericCovariantImpl, covariance.getPositionalVariance(1));
+  Expect.equals(0, covariance.getPositionalVariance(2));
+  Expect.equals(Covariance.Covariant | Covariance.GenericCovariantImpl,
+      covariance.getPositionalVariance(3));
+  Expect.equals(0, covariance.getPositionalVariance(4));
+
+  Expect.equals(Covariance.Covariant, covariance.getNamedVariance('a'));
+  Expect.equals(
+      Covariance.GenericCovariantImpl, covariance.getNamedVariance('b'));
+  Expect.equals(0, covariance.getNamedVariance('c'));
+  Expect.equals(Covariance.Covariant | Covariance.GenericCovariantImpl,
+      covariance.getNamedVariance('d'));
+  Expect.equals(0, covariance.getNamedVariance('e'));
+
+  Expect.isFalse(covariance.isTypeParameterGenericCovariantImpl(0));
+  Expect.isTrue(covariance.isTypeParameterGenericCovariantImpl(1));
+  Expect.isFalse(covariance.isTypeParameterGenericCovariantImpl(0));
+
+  Expect.stringEquals(
+      'Covariance('
+      '0:Covariant,1:GenericCovariantImpl,3:GenericCovariantImpl+Covariant,'
+      'a:Covariant,b:GenericCovariantImpl,d:GenericCovariantImpl+Covariant,'
+      'types:1)',
+      covariance.toString());
+
+  Procedure noParameterProcedure =
+      new Procedure(null, ProcedureKind.Method, new FunctionNode(null));
+  Covariance noParameterProcedureCovariance =
+      new Covariance.fromMember(noParameterProcedure, forSetter: false);
+  Expect.isTrue(noParameterProcedureCovariance.isEmpty);
+
+  covariance.applyCovariance(noParameterProcedure);
+  noParameterProcedureCovariance =
+      new Covariance.fromMember(noParameterProcedure, forSetter: false);
+  Expect.isTrue(noParameterProcedureCovariance.isEmpty);
+
+  Procedure oneParameterProcedure = new Procedure(
+      null,
+      ProcedureKind.Method,
+      new FunctionNode(null,
+          positionalParameters: [new VariableDeclaration(null)]));
+  Covariance oneParameterProcedureCovariance =
+      new Covariance.fromMember(oneParameterProcedure, forSetter: false);
+  Expect.isTrue(oneParameterProcedureCovariance.isEmpty);
+
+  covariance.applyCovariance(oneParameterProcedure);
+  oneParameterProcedureCovariance =
+      new Covariance.fromMember(oneParameterProcedure, forSetter: false);
+  Expect.isFalse(oneParameterProcedureCovariance.isEmpty);
+  Expect.equals(new Covariance.internal([Covariance.Covariant], null, null),
+      oneParameterProcedureCovariance);
+
+  Procedure positionalParametersProcedure = new Procedure(
+      null,
+      ProcedureKind.Method,
+      new FunctionNode(null, positionalParameters: [
+        new VariableDeclaration(null),
+        new VariableDeclaration(null),
+        new VariableDeclaration(null),
+        new VariableDeclaration(null),
+        new VariableDeclaration(null)
+      ]));
+  Covariance positionalParametersProcedureCovariance =
+      new Covariance.fromMember(positionalParametersProcedure,
+          forSetter: false);
+  Expect.isTrue(positionalParametersProcedureCovariance.isEmpty);
+
+  covariance.applyCovariance(positionalParametersProcedure);
+  positionalParametersProcedureCovariance = new Covariance.fromMember(
+      positionalParametersProcedure,
+      forSetter: false);
+  Expect.isFalse(positionalParametersProcedureCovariance.isEmpty);
+  checkEquals(
+      new Covariance.internal([
+        Covariance.Covariant,
+        Covariance.GenericCovariantImpl,
+        0,
+        Covariance.Covariant | Covariance.GenericCovariantImpl
+      ], null, null),
+      positionalParametersProcedureCovariance);
+
+  Procedure namedParametersProcedure = new Procedure(
+      null,
+      ProcedureKind.Method,
+      new FunctionNode(null, namedParameters: [
+        new VariableDeclaration('a'),
+        new VariableDeclaration('b'),
+        new VariableDeclaration('c'),
+        new VariableDeclaration('d'),
+        new VariableDeclaration('e')
+      ]));
+  Covariance namedParametersProcedureCovariance =
+      new Covariance.fromMember(namedParametersProcedure, forSetter: false);
+  Expect.isTrue(namedParametersProcedureCovariance.isEmpty);
+
+  covariance.applyCovariance(namedParametersProcedure);
+  namedParametersProcedureCovariance =
+      new Covariance.fromMember(namedParametersProcedure, forSetter: false);
+  Expect.isFalse(namedParametersProcedureCovariance.isEmpty);
+  checkEquals(
+      new Covariance.internal(
+          null,
+          {
+            'a': Covariance.Covariant,
+            'b': Covariance.GenericCovariantImpl,
+            'd': Covariance.Covariant | Covariance.GenericCovariantImpl
+          },
+          null),
+      namedParametersProcedureCovariance);
+
+  Procedure typeParametersProcedure = new Procedure(
+      null,
+      ProcedureKind.Method,
+      new FunctionNode(null, typeParameters: [
+        new TypeParameter(null),
+        new TypeParameter(null),
+        new TypeParameter(null),
+      ]));
+  Covariance typeParametersProcedureCovariance =
+      new Covariance.fromMember(typeParametersProcedure, forSetter: false);
+  Expect.isTrue(typeParametersProcedureCovariance.isEmpty);
+
+  covariance.applyCovariance(typeParametersProcedure);
+  typeParametersProcedureCovariance =
+      new Covariance.fromMember(typeParametersProcedure, forSetter: false);
+  Expect.isFalse(typeParametersProcedureCovariance.isEmpty);
+  checkEquals(new Covariance.internal(null, null, [false, true]),
+      typeParametersProcedureCovariance);
+
+  Covariance merged =
+      const Covariance.empty().merge(positionalParametersProcedureCovariance);
+  checkEquals(positionalParametersProcedureCovariance, merged);
+  merged = merged.merge(namedParametersProcedureCovariance);
+  checkEquals(
+      new Covariance.internal([
+        Covariance.Covariant,
+        Covariance.GenericCovariantImpl,
+        0,
+        Covariance.GenericCovariantImpl | Covariance.Covariant
+      ], {
+        'a': Covariance.Covariant,
+        'b': Covariance.GenericCovariantImpl,
+        'd': Covariance.Covariant | Covariance.GenericCovariantImpl
+      }, null),
+      merged);
+  merged = merged.merge(typeParametersProcedureCovariance);
+  checkEquals(covariance, merged);
+}
diff --git a/pkg/front_end/test/spell_checking_list_code.txt b/pkg/front_end/test/spell_checking_list_code.txt
index 1b3dce825f3f..65dc5999f29e 100644
--- a/pkg/front_end/test/spell_checking_list_code.txt
+++ b/pkg/front_end/test/spell_checking_list_code.txt
@@ -514,6 +514,7 @@ il
 imitate
 immutability
 impl
+impl+
 implementers
 imply
 implying
@@ -592,8 +593,8 @@ l
 lacks
 lang
 largest
-launcher
 lattice
+launcher
 layer
 layout
 lc
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart
new file mode 100644
index 000000000000..7f60cbd65852
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart
@@ -0,0 +1,17 @@
+// Copyright (c) 2020, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+// @dart=2.9
+
+import 'main_lib.dart';
+
+class E1 with A, D {}
+
+class E2 = Object with A, D;
+
+abstract class C6 extends C3 implements C4 {}
+
+abstract class C8 extends C5 implements C7 {}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.textual_outline.expect b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.textual_outline.expect
new file mode 100644
index 000000000000..1763bc0bb304
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.textual_outline.expect
@@ -0,0 +1,12 @@
+// @dart = 2.9
+import 'main_lib.dart';
+
+class E1 with A, D {}
+
+class E2 = Object with A, D;
+
+abstract class C6 extends C3 implements C4 {}
+
+abstract class C8 extends C5 implements C7 {}
+
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.textual_outline_modelled.expect b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.textual_outline_modelled.expect
new file mode 100644
index 000000000000..9bd7263aa3b5
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.textual_outline_modelled.expect
@@ -0,0 +1,11 @@
+// @dart = 2.9
+import 'main_lib.dart';
+
+abstract class C6 extends C3 implements C4 {}
+
+abstract class C8 extends C5 implements C7 {}
+
+class E1 with A, D {}
+
+class E2 = Object with A, D;
+main() {}
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.weak.expect b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.weak.expect
new file mode 100644
index 000000000000..1345fddd3196
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.weak.expect
@@ -0,0 +1,156 @@
+library;
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract class _E1&Object&A = core::Object with mai::A /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E1&Object&A*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+abstract class _E1&Object&A&D = self::_E1&Object&A with mai::D /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E1&Object&A&D*
+    : super self::_E1&Object&A::•()
+    ;
+  abstract forwarding-stub method method(covariant core::num* i, {core::String* s = #C1}) → core::String*;
+}
+class E1 extends self::_E1&Object&A&D {
+  synthetic constructor •() → self::E1*
+    : super self::_E1&Object&A&D::•()
+    ;
+  abstract member-signature method method(covariant core::num* i, {core::String* s = #C1}) → core::String*; -> self::_E1&Object&A&D::method
+}
+abstract class _E2&Object&A = core::Object with mai::A /*isAnonymousMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E2&Object&A*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+class E2 = self::_E2&Object&A with mai::D /*hasConstConstructor*/  {
+  const synthetic constructor •() → self::E2*
+    : super self::_E2&Object&A::•()
+    ;
+  abstract forwarding-stub method method(covariant core::num* i, {core::String* s = #C1}) → core::String*;
+}
+abstract class C6 extends mai::C3 implements mai::C4 {
+  synthetic constructor •() → self::C6*
+    : super mai::C3::•()
+    ;
+  abstract forwarding-stub method method2([covariant core::String* a = #C2]) → dynamic;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+abstract class C8 extends mai::C5 implements mai::C7 {
+  synthetic constructor •() → self::C8*
+    : super mai::C5::•()
+    ;
+  abstract forwarding-stub method method2([covariant core::String* a = #C2, core::num* b = #C2]) → dynamic;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+static method main() → dynamic {}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+abstract class Interface extends core::Object {
+  synthetic constructor •() → mai::Interface
+    : super core::Object::•()
+    ;
+  abstract method method(core::num i) → core::String;
+}
+abstract class Interface2 extends core::Object {
+  synthetic constructor •() → mai::Interface2
+    : super core::Object::•()
+    ;
+  abstract method method(covariant core::int i) → core::String;
+}
+abstract class A extends core::Object implements mai::Interface /*isMixinDeclaration*/  {
+  method method(core::num i, {core::String s = #C1}) → core::String
+    return s;
+}
+abstract class D extends core::Object implements mai::Interface, mai::Interface2 {
+  synthetic constructor •() → mai::D
+    : super core::Object::•()
+    ;
+  abstract forwarding-stub method method(covariant core::num i) → core::String;
+}
+abstract class C1 extends core::Object {
+  synthetic constructor •() → mai::C1
+    : super core::Object::•()
+    ;
+  abstract method method2() → dynamic;
+}
+abstract class C2 extends core::Object {
+  synthetic constructor •() → mai::C2
+    : super core::Object::•()
+    ;
+  abstract method method2([core::String a = #C2]) → dynamic;
+}
+abstract class C3 extends core::Object implements mai::C1, mai::C2 {
+  synthetic constructor •() → mai::C3
+    : super core::Object::•()
+    ;
+  abstract member-signature method method2([core::String a = #C2]) → dynamic; -> mai::C2::method2
+}
+abstract class C4 extends core::Object {
+  synthetic constructor •() → mai::C4
+    : super core::Object::•()
+    ;
+  abstract method method2([covariant core::String a = #C2]) → dynamic;
+}
+abstract class C5 extends mai::C3 implements mai::C4 {
+  synthetic constructor •() → mai::C5
+    : super mai::C3::•()
+    ;
+  abstract forwarding-stub method method2([covariant core::String a = #C2]) → dynamic;
+}
+abstract class C7 extends core::Object {
+  synthetic constructor •() → mai::C7
+    : super core::Object::•()
+    ;
+  abstract method method2([core::String a = #C2, core::num b = #C2]) → dynamic;
+}
+
+constants  {
+  #C1 = "hello"
+  #C2 = null
+}
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.weak.transformed.expect b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.weak.transformed.expect
new file mode 100644
index 000000000000..341bd9b9cf78
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/main.dart.weak.transformed.expect
@@ -0,0 +1,160 @@
+library;
+import self as self;
+import "dart:core" as core;
+import "main_lib.dart" as mai;
+
+import "org-dartlang-testcase:///main_lib.dart";
+
+abstract class _E1&Object&A extends core::Object implements mai::A /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E1&Object&A*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+  method /*isNonNullableByDefault, from org-dartlang-testcase:///main_lib.dart */ method(core::num i, {core::String s = #C1}) → core::String
+    return s;
+}
+abstract class _E1&Object&A&D extends self::_E1&Object&A implements mai::D /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E1&Object&A&D*
+    : super self::_E1&Object&A::•()
+    ;
+  abstract forwarding-stub method method(covariant core::num* i, {core::String* s = #C1}) → core::String*;
+}
+class E1 extends self::_E1&Object&A&D {
+  synthetic constructor •() → self::E1*
+    : super self::_E1&Object&A&D::•()
+    ;
+  abstract member-signature method method(covariant core::num* i, {core::String* s = #C1}) → core::String*; -> self::_E1&Object&A&D::method
+}
+abstract class _E2&Object&A extends core::Object implements mai::A /*isAnonymousMixin,isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::_E2&Object&A*
+    : super core::Object::•()
+    ;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+  method /*isNonNullableByDefault, from org-dartlang-testcase:///main_lib.dart */ method(core::num i, {core::String s = #C1}) → core::String
+    return s;
+}
+class E2 extends self::_E2&Object&A implements mai::D /*isEliminatedMixin,hasConstConstructor*/  {
+  const synthetic constructor •() → self::E2*
+    : super self::_E2&Object&A::•()
+    ;
+  abstract forwarding-stub method method(covariant core::num* i, {core::String* s = #C1}) → core::String*;
+}
+abstract class C6 extends mai::C3 implements mai::C4 {
+  synthetic constructor •() → self::C6*
+    : super mai::C3::•()
+    ;
+  abstract forwarding-stub method method2([covariant core::String* a = #C2]) → dynamic;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+abstract class C8 extends mai::C5 implements mai::C7 {
+  synthetic constructor •() → self::C8*
+    : super mai::C5::•()
+    ;
+  abstract forwarding-stub method method2([covariant core::String* a = #C2, core::num* b = #C2]) → dynamic;
+  abstract member-signature get _identityHashCode() → core::int*; -> core::Object::_identityHashCode
+  abstract member-signature method _instanceOf(dynamic instantiatorTypeArguments, dynamic functionTypeArguments, dynamic type) → core::bool*; -> core::Object::_instanceOf
+  abstract member-signature method _simpleInstanceOf(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOf
+  abstract member-signature method _simpleInstanceOfTrue(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfTrue
+  abstract member-signature method _simpleInstanceOfFalse(dynamic type) → core::bool*; -> core::Object::_simpleInstanceOfFalse
+  abstract member-signature operator ==(dynamic other) → core::bool*; -> core::Object::==
+  abstract member-signature get hashCode() → core::int*; -> core::Object::hashCode
+  abstract member-signature method toString() → core::String*; -> core::Object::toString
+  abstract member-signature method noSuchMethod(core::Invocation* invocation) → dynamic; -> core::Object::noSuchMethod
+  abstract member-signature get runtimeType() → core::Type*; -> core::Object::runtimeType
+}
+static method main() → dynamic {}
+
+library /*isNonNullableByDefault*/;
+import self as mai;
+import "dart:core" as core;
+
+abstract class Interface extends core::Object {
+  synthetic constructor •() → mai::Interface
+    : super core::Object::•()
+    ;
+  abstract method method(core::num i) → core::String;
+}
+abstract class Interface2 extends core::Object {
+  synthetic constructor •() → mai::Interface2
+    : super core::Object::•()
+    ;
+  abstract method method(covariant core::int i) → core::String;
+}
+abstract class A extends core::Object implements mai::Interface /*isMixinDeclaration*/  {
+  method method(core::num i, {core::String s = #C1}) → core::String
+    return s;
+}
+abstract class D extends core::Object implements mai::Interface, mai::Interface2 {
+  synthetic constructor •() → mai::D
+    : super core::Object::•()
+    ;
+  abstract forwarding-stub method method(covariant core::num i) → core::String;
+}
+abstract class C1 extends core::Object {
+  synthetic constructor •() → mai::C1
+    : super core::Object::•()
+    ;
+  abstract method method2() → dynamic;
+}
+abstract class C2 extends core::Object {
+  synthetic constructor •() → mai::C2
+    : super core::Object::•()
+    ;
+  abstract method method2([core::String a = #C2]) → dynamic;
+}
+abstract class C3 extends core::Object implements mai::C1, mai::C2 {
+  synthetic constructor •() → mai::C3
+    : super core::Object::•()
+    ;
+  abstract member-signature method method2([core::String a = #C2]) → dynamic; -> mai::C2::method2
+}
+abstract class C4 extends core::Object {
+  synthetic constructor •() → mai::C4
+    : super core::Object::•()
+    ;
+  abstract method method2([covariant core::String a = #C2]) → dynamic;
+}
+abstract class C5 extends mai::C3 implements mai::C4 {
+  synthetic constructor •() → mai::C5
+    : super mai::C3::•()
+    ;
+  abstract forwarding-stub method method2([covariant core::String a = #C2]) → dynamic;
+}
+abstract class C7 extends core::Object {
+  synthetic constructor •() → mai::C7
+    : super core::Object::•()
+    ;
+  abstract method method2([core::String a = #C2, core::num b = #C2]) → dynamic;
+}
+
+constants  {
+  #C1 = "hello"
+  #C2 = null
+}
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/main_lib.dart b/pkg/front_end/testcases/nnbd_mixed/issue43988/main_lib.dart
new file mode 100644
index 000000000000..52914147048c
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/main_lib.dart
@@ -0,0 +1,37 @@
+// Copyright (c) 2020, the Dart project authors. Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+abstract class Interface {
+  String method(num i);
+}
+
+abstract class Interface2 {
+  String method(covariant int i);
+}
+
+mixin A implements Interface {
+  String method(num i, {String s = "hello"}) => s;
+}
+
+abstract class D implements Interface, Interface2 {}
+
+abstract class C1 {
+  method2();
+}
+
+abstract class C2 {
+  method2([String a]);
+}
+
+abstract class C3 implements C1, C2 {}
+
+abstract class C4 {
+  method2([covariant String a]);
+}
+
+abstract class C5 extends C3 implements C4 {}
+
+abstract class C7 {
+  method2([String a, num b]);
+}
diff --git a/pkg/front_end/testcases/nnbd_mixed/issue43988/test.options b/pkg/front_end/testcases/nnbd_mixed/issue43988/test.options
new file mode 100644
index 000000000000..bfe6dc8532c2
--- /dev/null
+++ b/pkg/front_end/testcases/nnbd_mixed/issue43988/test.options
@@ -0,0 +1 @@
+main_lib.dart
\ No newline at end of file
diff --git a/pkg/kernel/lib/class_hierarchy.dart b/pkg/kernel/lib/class_hierarchy.dart
index 9f92ab0126ab..710baf7ec818 100644
--- a/pkg/kernel/lib/class_hierarchy.dart
+++ b/pkg/kernel/lib/class_hierarchy.dart
@@ -765,7 +765,12 @@ class ClosedWorldClassHierarchy implements ClassHierarchy {
   Member getDispatchTarget(Class class_, Name name, {bool setter: false}) {
     List<Member> list =
         _buildImplementedMembers(class_, infoFor(class_), setters: setter);
-    return ClassHierarchy.findMemberByName(list, name);
+    Member member = ClassHierarchy.findMemberByName(list, name);
+    assert(
+        member == null || !member.isAbstract,
+        "Abstract member $member found as dispatch target "
+        "for $name on $class_");
+    return member;
   }
 
   @override