Support for extending native classes

tests/compiler/dart2js/mock_compiler.dart

 // Copyright (c) 2012, 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 mock_compiler;
 
 import "package:expect/expect.dart";
 import 'dart:collection';
 
 import '../../../sdk/lib/_internal/compiler/compiler.dart' as api;
@@ skipping 146 matching lines @@
   }
   class JSBool extends Interceptor implements bool {
   }
   class JSFunction extends Interceptor implements Function {
   }
   class ObjectInterceptor {
   }
   getInterceptor(x) {}
   getNativeInterceptor(x) {}
   var dispatchPropertyName;
+  var mapTypeToInterceptor;
   getDispatchProperty(o) {}
   initializeDispatchProperty(f,p,i) {}
   initializeDispatchPropertyCSP(f,p,i) {}
 ''';
 
 const String DEFAULT_CORELIB = r'''
   print(var obj) {}
   abstract class num {}
   abstract class int extends num { }
   abstract class double extends num {
@@ skipping 278 matching lines @@
   }
 }
 
 class MockDeferredLoadTask extends DeferredLoadTask {
   MockDeferredLoadTask(Compiler compiler) : super(compiler);
 
   void registerMainApp(LibraryElement mainApp) {
     // Do nothing.
   }
 }