Fixes #378

lib/src/codegen/js_field_storage.dart

 // Copyright (c) 2015, 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.
 
 library dev_compiler.src.codegen.js_field_storage;
 
 import 'dart:collection' show HashMap, HashSet;
 import 'package:analyzer/src/generated/ast.dart' show Identifier;
 import 'package:analyzer/src/generated/element.dart';
 
 import '../info.dart' show LibraryUnit;
 
 /// We use a storage slot for fields that override or can be overridden by
 /// getter/setter pairs.
-HashSet<FieldElement> findFieldsNeedingStorage(LibraryUnit library) {
+HashSet<FieldElement> findFieldsNeedingStorage(
+    LibraryUnit library, HashSet<ClassElement> extensionTypes) {
   var overrides = new HashSet<FieldElement>();
   for (var unit in library.partsThenLibrary) {
     for (var cls in unit.element.types) {
       var superclasses = getSuperclasses(cls);
       for (var field in cls.fields) {
         if (!field.isSynthetic && !overrides.contains(field)) {
-          checkForPropertyOverride(field, superclasses, overrides);
+          checkForPropertyOverride(
+              field, superclasses, overrides, extensionTypes);
         }
       }
     }
   }
 
   return overrides;
 }
 
-void checkForPropertyOverride(FieldElement field,
-    List<ClassElement> superclasses, HashSet<FieldElement> overrides) {
+void checkForPropertyOverride(
+    FieldElement field,
+    List<ClassElement> superclasses,
+    HashSet<FieldElement> overrides,
+    HashSet<ClassElement> extensionTypes) {
   assert(!field.isSynthetic);
 
   var library = field.library;
 
   bool found = false;
   for (var superclass in superclasses) {
     var superprop = getProperty(superclass, library, field.name);
     if (superprop != null) {
       // If we find an abstract getter/setter pair, stop the search.
       var getter = superprop.getter;
       var setter = superprop.setter;
-      if ((getter == null || getter.isAbstract) &&
+      if (!extensionTypes.contains(superclass) &&
+          (getter == null || getter.isAbstract) &&
           (setter == null || setter.isAbstract)) {
         break;
       }
 
       found = true;
       // Record that the super property is overridden.
       if (superprop.library == library) overrides.add(superprop);
     }
   }
 
@@ skipping 23 matching lines @@
       if (mixin != null) result.add(mixin);
     }
     var supertype = cls.supertype;
     if (supertype == null) break;
 
     cls = supertype.element;
     result.add(cls);
   }
   return result;
 }