Maintain referential integrity of proxy instances in dart:js

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 290a0fbb85e5b77cdcd43b4cacc06109668ef362..3578887f24cda3ca0a5d035ce1fe8bd76557ae14 100644
--- a/tests/html/js_test.dart
+++ b/tests/html/js_test.dart
@@ -157,6 +157,18 @@ main() {
     expect(identical(c1, c2), isTrue);
   });
 
+  test('identical JS objects should have identical proxies', () {
+    var o1 = context['location'];
+    var o2 = context['location'];
+    expect(identical(o1, o2), isTrue);
+  });
+
+  test('identical JS functions should have identical proxies', () {
+    var f1 = context['Object'];
+    var f2 = context['Object'];
+    expect(identical(f1, f2), isTrue);
+  });
+
   test('read global field', () {
     expect(context['x'], equals(42));
     expect(context['y'], isNull);