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

sdk/lib/js/dartium/js_dartium.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.
 
 /**
  * The js.dart library provides simple JavaScript invocation from Dart that
  * works on both Dartium and on other modern browsers via Dart2JS.
  *
  * It provides a model based on scoped [JsObject] objects.  Proxies give Dart
  * code access to JavaScript objects, fields, and functions as well as the
@@ skipping 58 matching lines @@
 import 'dart:html';
 import 'dart:isolate';
 
 // Global ports to manage communication from Dart to JS.
 SendPortSync _jsPortSync = window.lookupPort('dart-js-context');
 SendPortSync _jsPortCreate = window.lookupPort('dart-js-create');
 SendPortSync _jsPortInstanceof = window.lookupPort('dart-js-instanceof');
 SendPortSync _jsPortDeleteProperty = window.lookupPort('dart-js-delete-property');
 SendPortSync _jsPortConvert = window.lookupPort('dart-js-convert');
 
+
+JsObject _context;
+
 /**
  * Returns a proxy to the global JavaScript context for this page.
  */
 JsObject get context {
-  var port = _jsPortSync;
-  if (port == null) {
-    return null;
+  if (_context == null) {
+    var port = _jsPortSync;
+    if (port == null) {
+      return null;
+    }
+    _context = _deserialize(_jsPortSync.callSync([]));
   }
-  return _deserialize(_jsPortSync.callSync([]));
+  return _context;
 }
 
 /**
  * Converts a json-like [data] to a JavaScript map or array and return a
  * [JsObject] to it.
  */
 JsObject jsify(dynamic data) => data == null ? null : new JsObject._json(data);
 
 /**
  * Converts a local Dart function to a callback that can be passed to
@@ skipping 271 matching lines @@
     // Serialized type.
     return message;
   }
   var tag = message[0];
   switch (tag) {
     case 'funcref': return deserializeFunction(message);
     case 'objref': return deserializeObject(message);
   }
   throw 'Unsupported serialized data: $message';
 }