Adding tests for accessing custom elements from alternate script contexts

tests/html/custom/js_custom_test.dart

Index: tests/html/custom/js_custom_test.dart
diff --git a/tests/html/custom/js_custom_test.dart b/tests/html/custom/js_custom_test.dart
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+// 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 js_custom_test;
+
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+import '../utils.dart';
+import 'dart:mirrors';
+
+class A extends HtmlElement {
+  static final tag = 'x-a';
+  factory A() => new Element.tag(tag);
+  A.created() : super.created() {
+    ncallbacks++;
+  }
+
+  static int ncallbacks = 0;
+}
+
+main() {
+  useHtmlConfiguration();
+
+  // Adapted from Blink's
+  // fast/dom/custom/constructor-calls-created-synchronously test.
+
+  var registered = false;
+  setUp(() {
+    return loadPolyfills().then((_) {
+      if (!registered) {
+        registered = true;
+        document.register(A.tag, A);
+      }
+    });
+  });
+
+  test('accessing custom Dart element from JS', () {
+    var a = new A();
+    a.id = 'a';
+    document.body.append(a);
+
+    var script = '''
+      document.querySelector('#a').setAttribute('fromJS', 'true');
+    ''';
+    document.body.append(new ScriptElement()..text = script);
+
+    expect(a.attributes['fromJS'], 'true');
+  });
+
+  test('accessing custom JS element from Dart', () {
+    var script = '''
+    var Foo = document.register('x-foo', {
+      prototype: Object.create(HTMLElement.prototype, {
+        createdCallback: {
+          value: function() {
+            window.console.log('Foo created!');
+            this.setAttribute('fromJS', 'true');
+          }
+        }
+      })});
+    var foo = new Foo();
+    foo.id = 'b';
+    document.body.appendChild(foo);
+    ''';
+
+    document.body.append(new ScriptElement()..text = script);
+    var custom = document.querySelector('#b');
+    expect(custom is HtmlElement, isTrue);
+    expect(custom.attributes['fromJS'], 'true');
+  });
+}