Support user generated custom native JS classes.

tests/html/mirrors_js_typed_interop_test.dart

Index: tests/html/mirrors_js_typed_interop_test.dart
diff --git a/tests/html/mirrors_js_typed_interop_test.dart b/tests/html/mirrors_js_typed_interop_test.dart
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+++ b/tests/html/mirrors_js_typed_interop_test.dart
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+// Copyright (c) 2015, 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_js_typed_interop_test;
+
+import 'dart:mirrors';
+import 'dart:html';
+
+import 'package:js/js.dart';
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+
+_injectJs() {
+  document.body.append(new ScriptElement()
+    ..type = 'text/javascript'
+    ..innerHtml = r"""
+  window.foo = {
+    x: 3,
+    z: 100,
+    multiplyBy2: function(arg) { return arg * 2; },
+  };
+""");
+}
+
+@Js()
+external Foo get foo;
+
+@Js()
+class Foo {
+  external int get x;
+  external int set x(v);
+  external num multiplyBy2(num y);
+}
+
+main() {
+  _injectJs();
+
+  useHtmlConfiguration();
+
+  test('dynamic dispatch', () {
+    var f = foo;
+    expect(f.x, 3);
+    // JsInterop methods are not accessible using reflection.
+    expect(() => reflect(f).setField(#x, 123), throws);
+    expect(f.x, 3);
+  });
+}