Avoid throwing exceptions on inter-frame communication

dart/sdk/lib/js/dart2js/js_dart2js.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.
 
 /**
  * Support for interoperating with JavaScript.
  *
  * This library provides access to JavaScript objects from Dart, allowing
  * Dart code to get and set properties, and call methods of JavaScript objects
  * and invoke JavaScript functions. The library takes care of converting
@@ skipping 453 matching lines @@
 final String _DART_OBJECT_PROPERTY_NAME =
     getIsolateAffinityTag(r'_$dart_dartObject');
 final String _DART_CLOSURE_PROPERTY_NAME =
     getIsolateAffinityTag(r'_$dart_dartClosure');
 
 // property added to a JS object referencing its Dart-side JsObject proxy
 const _JS_OBJECT_PROPERTY_NAME = r'_$dart_jsObject';
 const _JS_FUNCTION_PROPERTY_NAME = r'$dart_jsFunction';
 
 bool _defineProperty(o, String name, value) {
-  if (JS('bool', 'Object.isExtensible(#)', o)) {
+  if (_isExtensible(o) &&
+      // TODO(ahe): Calling _hasOwnProperty to work around
+      // https://code.google.com/p/dart/issues/detail?id=21331.
+      !_hasOwnProperty(o, name)) {
     try {
       JS('void', 'Object.defineProperty(#, #, { value: #})', o, name, value);
       return true;
-    } catch(e) {
+    } catch (e) {
       // object is native and lies about being extensible
       // see https://bugzilla.mozilla.org/show_bug.cgi?id=775185
     }
   }
   return false;
 }
 
+bool _hasOwnProperty(o, String name) {
+  return JS('bool', 'Object.prototype.hasOwnProperty.call(#, #)', o, name);
+}
+
+bool _isExtensible(o) => JS('bool', 'Object.isExtensible(#)', o);
+
 Object _getOwnProperty(o, String name) {
-  if (JS('bool', 'Object.prototype.hasOwnProperty.call(#, #)', o, name)) {
+  if (_hasOwnProperty(o, name)) {
     return JS('', '#[#]', o, name);
   }
   return null;
 }
 
 bool _isLocalObject(o) => JS('bool', '# instanceof Object', o);
 
 // The shared constructor function for proxies to Dart objects in JavaScript.
 final _dartProxyCtor = JS('', 'function DartObject(o) { this.o = o; }');
 
@@ skipping 77 matching lines @@
   // Dart in that context. The JS object will have a cached proxy
   // but it won't be a valid Dart object in this context.
   // For now we throw away the cached proxy, but we should be able
   // to cache proxies from multiple JS contexts and Dart isolates.
   if (dartProxy == null || !_isLocalObject(o)) {
     dartProxy = createProxy(o);
     _defineProperty(o, propertyName, dartProxy);
   }
   return dartProxy;
 }