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

sdk/lib/js/dart2js/js_dart2js.dart

Index: sdk/lib/js/dart2js/js_dart2js.dart
diff --git a/sdk/lib/js/dart2js/js_dart2js.dart b/sdk/lib/js/dart2js/js_dart2js.dart
index 7f806b73f3744794eeca4b725e61093b6cf12008..9fc956c808dad5cd7700bf9f1dc7d026fd485c7c 100644
--- a/sdk/lib/js/dart2js/js_dart2js.dart
+++ b/sdk/lib/js/dart2js/js_dart2js.dart
@@ -5,11 +5,9 @@
 library dart.js;
 
 import 'dart:_foreign_helper' show JS;
-import 'dart:_js_helper' show convertDartClosureToJS;
+import 'dart:_js_helper' show convertDartClosureToJS, computeGlobalThis;
 
-JsObject get context {
-  return new JsObject._fromJs(JS('=Object', 'window'));
-}
+final JsObject context = new JsObject._fromJs(computeGlobalThis());
 
 JsObject jsify(dynamic data) => data == null ? null : new JsObject._json(data);