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) && !_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 73 matching lines @@
   var dartProxy = _getOwnProperty(o, propertyName);
   // Temporary fix for dartbug.com/15193
   // In some cases it's possible to see a JavaScript object that
   // came from a different context and was previously proxied to
   // 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);

[ahe, 2014/10/13 11:36:23]

Perhaps the problem is here.

I'm using dart:js to work around a bug in dart:html which prevents me from
installing onerror handler on iframe.contentWindow. Both the main page, and the
iframe run dart2js output, and they both use
"___dart__$dart_dartObject_ZxYxX_0_" as the dispatch property.

Also, it appears that we always try to define the property on non-local objects.
This will throw on the second attempt (but since we swallow exceptions in
defineProperty this goes unnoticed.

I'm not attempting to fix this, someone more familiar with the code should look
at this. I'm simply trying to avoid throwing so many exceptions as it is making
debugging incremental compilation hard.

   }
   return dartProxy;
 }