Drop support for final, interface, and sealed mixins

lib/src/element_type.dart

@@ -101,7 +101,7 @@ class UndefinedElementType extends ElementType {
 
   @override
   String get name {
-    if (type.isVoid) return 'void';
+    if (type is VoidType) return 'void';
     if (type.isDynamic) return 'dynamic';
     assert(const {'Never'}.contains(typeElement!.name),
         'Unrecognized type for UndefinedElementType: ${type.toString()}');
@@ -436,7 +436,7 @@ class GenericTypeAliasElementType extends TypeParameterElementType {
 extension on DartType {
   /// The dartdoc nullability suffix for this type in [library].
   String nullabilitySuffixWithin(Library library) {
-    if (!isVoid && !isBottom) {
+    if (this is! VoidType && !isBottom) {
       /// If a legacy type appears inside the public interface of a Null
       /// safety library, we pretend it is nullable for the purpose of
       /// documentation (since star-types are not supposed to be public).

lib/src/generator/templates.runtime_renderers.dart

@@ -15982,6 +15982,7 @@ const _invisibleGetters = {
     'nonSynthetic',
     'runtimeType',
     'session',
+    'sinceSdkVersion',
     'source'
   },
   'ElementAnnotation': {
@@ -16074,6 +16075,7 @@ const _invisibleGetters = {
     'returnType',
     'runtimeType',
     'session',
+    'sinceSdkVersion',
     'substitution',
     'type',
     'typeParameters'
@@ -16314,6 +16316,7 @@ const _invisibleGetters = {
     'nonSynthetic',
     'runtimeType',
     'session',
+    'sinceSdkVersion',
     'substitution'
   },
   'MethodElement': {'augmentation', 'declaration', 'hashCode', 'runtimeType'},
@@ -16483,6 +16486,7 @@ const _invisibleGetters = {
     'parameters',
     'runtimeType',
     'session',
+    'sinceSdkVersion',
     'source',
     'substitution',
     'type',

lib/src/model/extension.dart

@@ -3,6 +3,7 @@
 // BSD-style license that can be found in the LICENSE file.
 
 import 'package:analyzer/dart/element/element.dart';
+import 'package:analyzer/dart/element/type.dart';
 import 'package:dartdoc/src/element_type.dart';
 import 'package:dartdoc/src/model/comment_referable.dart';
 import 'package:dartdoc/src/model/extension_target.dart';
@@ -23,7 +24,7 @@ class Extension extends Container implements EnclosedElement {
   /// Detect if this extension applies to every object.
   bool get alwaysApplies =>
       extendedType.instantiatedType.isDynamic ||
-      extendedType.instantiatedType.isVoid ||
+      extendedType.instantiatedType is VoidType ||
       extendedType.instantiatedType.isDartCoreObject;
 
   bool couldApplyTo<T extends ExtensionTarget>(T c) =>
@@ -32,7 +33,7 @@ class Extension extends Container implements EnclosedElement {
   /// Whether this extension could apply to [type].
   bool _couldApplyTo(DefinedElementType type) {
     if (extendedType.instantiatedType.isDynamic ||
-        extendedType.instantiatedType.isVoid) {
+        extendedType.instantiatedType is VoidType) {
       return true;
     }
     var typeInstantiated = type.instantiatedType;

lib/src/model/mixin.dart

@@ -61,18 +61,18 @@ class Mixin extends InheritingContainer with TypeImplementing {
   bool get isBase => element.isBase;
 
   @override
-  bool get isFinal => element.isFinal;
+  bool get isFinal => false;
 
   @override
-  bool get isInterface => element.isInterface;
+  bool get isInterface => false;
 
   @override
 
   /// Mixins are not mixin classes by definition.
   bool get isMixinClass => false;
 
   @override
-  bool get isSealed => element.isSealed;
+  bool get isSealed => false;
 
   @override
   String get kind => 'mixin';

test/class_modifiers_test.dart

@@ -45,9 +45,6 @@ abstract mixin class L {}
 abstract base mixin class M {}
 mixin N {}
 base mixin O {}
-interface mixin P {}
-final mixin Q {}
-sealed mixin R {}
 ''');
     // This almost seems worth a map and loop, but leaving expanded for now for
     // test clarity.
@@ -66,9 +63,6 @@ sealed mixin R {}
     var Mclass = library.classes.named('M');
     var Nmixin = library.mixins.named('N');
     var Omixin = library.mixins.named('O');
-    var Pmixin = library.mixins.named('P');
-    var Qmixin = library.mixins.named('Q');
-    var Rmixin = library.mixins.named('R');
     expect(Aclass.fullkind, equals('class'));
     expect(Bclass.fullkind, equals('base class'));
     expect(Cclass.fullkind, equals('interface class'));
@@ -84,8 +78,5 @@ sealed mixin R {}
     expect(Mclass.fullkind, equals('abstract base mixin class'));
     expect(Nmixin.fullkind, equals('mixin'));
     expect(Omixin.fullkind, equals('base mixin'));
-    expect(Pmixin.fullkind, equals('interface mixin'));
-    expect(Qmixin.fullkind, equals('final mixin'));
-    expect(Rmixin.fullkind, equals('sealed mixin'));
   }
 }