fix for #520, ListMixin should work again

lib/src/compiler/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.
 
 import 'dart:collection' show HashSet;
 
 import 'package:analyzer/dart/ast/ast.dart' show Identifier;
 import 'package:analyzer/dart/element/element.dart';
 
 import 'extension_types.dart';
 
 class PropertyOverrideResult {
   final bool foundGetter;
   final bool foundSetter;
 
   PropertyOverrideResult(this.foundGetter, this.foundSetter);
 }
 
 PropertyOverrideResult checkForPropertyOverride(FieldElement field,
     List<ClassElement> superclasses, ExtensionTypeSet extensionTypes) {
   bool foundGetter = false;
   bool foundSetter = false;
 
   for (var superclass in superclasses) {
     // Stop if we reach a native type.
-    if (extensionTypes.contains(superclass)) break;
+    if (extensionTypes.isNativeClass(superclass)) break;
 
     var superprop = getProperty(superclass, field.library, field.name);
     if (superprop == null) continue;
 
     // Static fields can override superclass static fields. However, we need to
     // handle the case where they override a getter or setter.
     if (field.isStatic && !superprop.isSynthetic) continue;
 
     var getter = superprop.getter;
     bool hasGetter = getter != null && !getter.isAbstract;
@@ skipping 33 matching lines @@
       if (mixin != null) result.add(mixin);
     }
     var supertype = cls.supertype;
     if (supertype == null) break;
 
     cls = supertype.element;
     result.add(cls);
   }
   return result;
 }