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

tests/html/js_test.dart

Index: tests/html/js_test.dart
diff --git a/tests/html/js_test.dart b/tests/html/js_test.dart
index ff6753d3690d04720c6db871d34689f8e88bcde0..290a0fbb85e5b77cdcd43b4cacc06109668ef362 100644
--- a/tests/html/js_test.dart
+++ b/tests/html/js_test.dart
@@ -151,6 +151,12 @@ main() {
   _injectJs();
   useHtmlConfiguration();
 
+  test('context instances should be identical', () {
+    var c1 = context;
+    var c2 = context;
+    expect(identical(c1, c2), isTrue);
+  });
+
   test('read global field', () {
     expect(context['x'], equals(42));
     expect(context['y'], isNull);