Fix for dart2js mirrors caching with custom elements

tests/html/custom/mirrors_test.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.
+
+library tests.html.mirrors_test;
+
+import 'dart:mirrors';
+import 'dart:html';
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import '../utils.dart';
+
+/// Regression test for a tricky mirrors+custom_elements issue:
+/// dart2js mirrors cache dispatch information on the Object's constructor.
+/// This was failing for custom elements on IE 10, because the constructor was
+/// HTMLUnknownElement for all of them. So mirrors called the wrong method.
+main() {
+  useHtmlConfiguration();
+
+  var registered = false;
+  setUp(() => loadPolyfills().then((_) {
+    if (!registered) {
+      registered = true;
+      document.register(A.tag, A);
+      document.register(B.tag, B);
+    }
+  }));
+
+  test('dynamic dispatch', () {
+    var a = new A();
+    expect(a.fooBar, 1);
+    reflect(a).setField(#fooBar, 123);
+    expect(a.fooBar, 123);
+
+    // Even though A was set first, B.fooBar= should dispatch to B.
+    var b = new B();
+    expect(b.fooBar, 2);
+    expect(b._fooBarSet, 0);
+    reflect(b).setField(#fooBar, 123);
+    expect(b.fooBar, 123);
+    expect(b._fooBarSet, 1);
+  });
+}
+
+class A extends HtmlElement {
+  static final tag = 'x-a';
+  factory A() => new Element.tag(tag);
+  A.created() : super.created();
+
+  int fooBar = 1;
+}
+
+class B extends HtmlElement {
+  static final tag = 'x-b';
+  factory B() => new Element.tag(tag);
+  B.created() : super.created();
+
+  int _fooBar = 2;
+  int _fooBarSet = 0;
+
+  int get fooBar => _fooBar;
+  set fooBar(value) {
+    _fooBarSet++;
+    _fooBar = value;
+  }
+}