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..ccade724fd302edef8fd493d5678c6857adeb2bb 100644
--- a/tests/html/js_test.dart
+++ b/tests/html/js_test.dart
@@ -124,6 +124,10 @@ function Baz(p1,p2,p3,p4,p5,p6,p7,p8,p9,p10,p11) {
   this.f10 = p10;
   this.f11 = p11;
 }
+
+function identical(o1, o2) {
+  return o1 === o2;
+}
 """;
   document.body.append(script);
 }
@@ -147,6 +151,8 @@ class Color implements Serializable<String> {
   String toJs() => this._value;
 }
 
+class TestDartObject {}
+
 main() {
   _injectJs();
   useHtmlConfiguration();
@@ -157,6 +163,28 @@ 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('identical Dart objects should have identical proxies', () {
+    var o1 = new TestDartObject();
+    expect(context.callMethod('identical', [o1, o1]), isTrue);
+  });
+
+  test('identical Dart functions should have identical proxies', () {
+    var f1 = () => print("I'm a Function!");
+    expect(context.callMethod('identical', [f1, f1]), isTrue);
+  });
+
   test('read global field', () {
     expect(context['x'], equals(42));
     expect(context['y'], isNull);