Adding 'supported' check for shadow root.

tests/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('../../pkg/unittest/unittest.dart');
+#import('../../pkg/unittest/html_config.dart');
+#import('dart:html');
+
+main() {
+  useHtmlConfiguration();
+
+  test('ShadowRoot supported', () {
+    var isSupported = ShadowRoot.supported;
+
+    // If it's supported, then it should work. Otherwise should fail.
+    if (isSupported) {
+      var shadowRoot = new ShadowRoot(new DivElement());
+      expect(shadowRoot is ShadowRoot);
+    } else {
+      expect(() => new ShadowRoot(new DivElement()), throws);
+    }
+  });
+}