Polyfill only once

pkg/dev_compiler/tool/input_sdk/private/js_helper.dart

 // Copyright (c) 2013, 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 dart._js_helper;
 
 import 'dart:collection';
 
 import 'dart:_debugger' show stackTraceMapper;
 
@@ skipping 892 matching lines @@
   Iterator<E> get iterator => new SyncIterator<E>(_jsIterator());
 }
 
 class BooleanConversionAssertionError extends AssertionError {
   toString() => 'Failed assertion: boolean expression must not be null';
 }
 
 // Hook to register new global object.  This is invoked from dart:html
 // whenever a new window is accessed for the first time.
 void registerGlobalObject(object) {
-  dart.polyfill(object);
-  dart.applyAllExtensions(object);
+  if (dart.polyfill(object)) {
+    dart.applyAllExtensions(object);
+  }
 }