Add all dart:html tests from the sdk to test/codegen.

test/codegen/lib/html/shadowroot_test.dart

+// Copyright (c) 2012, 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 ShadowRootTest;
+import 'package:unittest/unittest.dart';
+import 'package:unittest/html_config.dart';
+import 'dart:html';
+
+main() {
+  useHtmlConfiguration();
+
+  var isShadowRoot =
+      predicate((x) => x is ShadowRoot, 'is a ShadowRoot');
+
+  test('ShadowRoot supported', () {
+    var isSupported = ShadowRoot.supported;
+
+    // If it's supported, then it should work. Otherwise should fail.
+    if (isSupported) {
+      var div = new DivElement();
+      var shadowRoot = div.createShadowRoot();
+      expect(shadowRoot, isShadowRoot);
+      expect(div.shadowRoot, shadowRoot);
+    } else {
+      expect(() => new DivElement().createShadowRoot(), throws);
+    }
+  });
+}