Port custom element layout tests to Dart

tests/html/custom/constructor_calls_created_synchronously_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 constructor_calls_created_synchronously_test;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+
+class A extends HtmlElement {
+  static final tag = 'x-a';
+  factory A() => new Element.tag(tag);
+
+  static int ncallbacks = 0;
+
+  void onCreated() {
+    ncallbacks++;
+  }
+}
+
+main() {
+  useHtmlConfiguration();
+
+  // Adapted from Blink's
+  // fast/dom/custom/constructor-calls-created-synchronously test.
+
+  test('createdCallback', () {
+    document.register(A.tag, A);
+    var x = new A();
+    expect(A.ncallbacks, 1);
+  });
+}