diff --git a/pkg/dev_compiler/lib/runtime/dart/_interceptors.js b/pkg/dev_compiler/lib/runtime/dart/_interceptors.js
index 615c98079d6b..278ba256eb7b 100644
--- a/pkg/dev_compiler/lib/runtime/dart/_interceptors.js
+++ b/pkg/dev_compiler/lib/runtime/dart/_interceptors.js
@@ -487,6 +487,50 @@ dart.library('dart/_interceptors', null, /* Imports */[
   let _shrReceiverPositive = Symbol('_shrReceiverPositive');
   let _shrOtherPositive = Symbol('_shrOtherPositive');
   let _shrBothPositive = Symbol('_shrBothPositive');
+  dart.defineExtensionNames([
+    'compareTo',
+    'isNegative',
+    'isNaN',
+    'isInfinite',
+    'isFinite',
+    'remainder',
+    'abs',
+    'sign',
+    'toInt',
+    'truncate',
+    'ceil',
+    'floor',
+    'round',
+    'ceilToDouble',
+    'floorToDouble',
+    'roundToDouble',
+    'truncateToDouble',
+    'clamp',
+    'toDouble',
+    'toStringAsFixed',
+    'toStringAsExponential',
+    'toStringAsPrecision',
+    'toRadixString',
+    'toString',
+    'hashCode',
+    'unary-',
+    '+',
+    '-',
+    '/',
+    '*',
+    '%',
+    '~/',
+    '<<',
+    '>>',
+    '&',
+    '|',
+    '^',
+    '<',
+    '>',
+    '<=',
+    '>=',
+    'runtimeType'
+  ]);
   class JSNumber extends Interceptor {
     JSNumber() {
       super.Interceptor();
@@ -928,6 +972,9 @@ dart.library('dart/_interceptors', null, /* Imports */[
   });
   JSInt[dart.metadata] = () => [dart.const(new _js_helper.JsPeerInterface({name: 'Number'}))];
   dart.registerExtension(dart.global.Number, JSInt);
+  dart.defineExtensionNames([
+    'runtimeType'
+  ]);
   class JSDouble extends JSNumber {
     JSDouble() {
       super.JSNumber();
diff --git a/pkg/dev_compiler/lib/runtime/dart/core.js b/pkg/dev_compiler/lib/runtime/dart/core.js
index daddf3297142..9893f751f5ff 100644
--- a/pkg/dev_compiler/lib/runtime/dart/core.js
+++ b/pkg/dev_compiler/lib/runtime/dart/core.js
@@ -68,6 +68,9 @@ dart.library('dart/core', null, /* Imports */[
     constructors: () => ({_Proxy: [_Proxy, []]})
   });
   let proxy = dart.const(new _Proxy());
+  dart.defineExtensionNames([
+    'toString'
+  ]);
   class bool extends Object {
     static fromEnvironment(name, opts) {
       let defaultValue = opts && 'defaultValue' in opts ? opts.defaultValue : false;
@@ -1254,6 +1257,9 @@ dart.library('dart/core', null, /* Imports */[
     }
   }
   let Iterable$ = dart.generic(function(E) {
+    dart.defineExtensionNames([
+      'join'
+    ]);
     class Iterable extends Object {
       Iterable() {
       }
diff --git a/pkg/dev_compiler/lib/runtime/dart_runtime.js b/pkg/dev_compiler/lib/runtime/dart_runtime.js
index 3cb2c617ca74..1b6125ee3682 100644
--- a/pkg/dev_compiler/lib/runtime/dart_runtime.js
+++ b/pkg/dev_compiler/lib/runtime/dart_runtime.js
@@ -799,7 +799,12 @@ var dart, dartx;
     // Mark the JS type's instances so we can easily check for extensions.
     assert(jsProto[_extensionType] === void 0);
     jsProto[_extensionType] = extProto;
-    copyPropertiesHelper(jsProto, extProto, getOwnPropertySymbols(extProto));
+
+    let dartObjProto = core.Object.prototype;
+    while (extProto !== dartObjProto && extProto !== jsProto) {
+      copyPropertiesHelper(jsProto, extProto, getOwnPropertySymbols(extProto));
+      extProto = extProto.__proto__;
+    }
   }
   dart.registerExtension = registerExtension;
 
diff --git a/pkg/dev_compiler/lib/src/codegen/js_codegen.dart b/pkg/dev_compiler/lib/src/codegen/js_codegen.dart
index 99ece9c88e0c..a4f09d0a54d5 100644
--- a/pkg/dev_compiler/lib/src/codegen/js_codegen.dart
+++ b/pkg/dev_compiler/lib/src/codegen/js_codegen.dart
@@ -569,15 +569,17 @@ class JSCodegenVisitor extends GeneralizingAstVisitor {
     var name = classElem.name;
     var body = <JS.Statement>[];
 
-    if (jsPeerName != null) {
+    if (_extensionTypes.contains(classElem)) {
       var dartxNames = [];
       for (var m in methods) {
         if (!m.isAbstract && !m.isStatic && m.element.isPublic) {
           dartxNames.add(_elementMemberName(m.element, allowExtensions: false));
         }
       }
-      body.add(js.statement('dart.defineExtensionNames(#)',
-          [new JS.ArrayInitializer(dartxNames, multiline: true)]));
+      if (dartxNames.isNotEmpty) {
+        body.add(js.statement('dart.defineExtensionNames(#)',
+        [new JS.ArrayInitializer(dartxNames, multiline: true)]));
+      }
     }
 
     body.add(new JS.ClassDeclaration(cls));
diff --git a/pkg/dev_compiler/tool/input_sdk/private/js_number.dart b/pkg/dev_compiler/tool/input_sdk/private/js_number.dart
index 9d0b1b937310..ced02db358b9 100644
--- a/pkg/dev_compiler/tool/input_sdk/private/js_number.dart
+++ b/pkg/dev_compiler/tool/input_sdk/private/js_number.dart
@@ -343,6 +343,8 @@ class JSNumber extends Interceptor implements num {
  */
 // TODO(jmesserly): for dev_compiler all numbers will get `int` members at
 // runtime for dynamic dispatch. We can fix by checking it at dispatch time.
+// TODO(jmesserly): merge with JSNumber? That would simplify generated code,
+// and dart_runtime's extension mechanism.
 @JsPeerInterface(name: 'Number')
 class JSInt extends JSNumber implements int, double {
   const JSInt();