Cache the context proxy. Don't use window for global 'this'.

pkg/browser/lib/interop.js

 // 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 JS interoperability
 // ---------------------------------------------------------------------------
 function SendPortSync() {
 }
 
@@ skipping 206 matching lines @@
     window.addEventListener(source, listener, false);
     dispatchEvent(target, [source, serialized]);
     window.removeEventListener(source, listener, false);
     return deserialize(result);
   }
 })();
 
 (function() {
   // Proxy support for js.dart.
 
-  var globalContext = window;
+  // We don't use 'window' because we might be in a web worker, but we don't
+  // use 'self' because not all browsers support it
+  var globalContext = function() { return this; }();
 
   // Table for local objects and functions that are proxied.
   function ProxiedObjectTable() {
     // Name for debugging.
     this.name = 'js-ref';
 
     // Table from IDs to JS objects.
     this.map = {};
 
     // Generator for new IDs.
@@ skipping 292 matching lines @@
     port.receive(f);
     window.registerPort(name, port.toSendPort());
   }
 
   makeGlobalPort('dart-js-context', context);
   makeGlobalPort('dart-js-create', construct);
   makeGlobalPort('dart-js-instanceof', proxyInstanceof);
   makeGlobalPort('dart-js-delete-property', proxyDeleteProperty);
   makeGlobalPort('dart-js-convert', proxyConvert);
 })();