added shadow dom layout test

tests/html/shadow_dom_layout_test.dart

+// Copyright (c) 2011, 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('ShadowDOMTest');
+
+#import('dart:html');
+
+// Test that children of a shadow host get distributed properly to the
+// insertion points of a shadow subtree. Output should be three boxes,
+// ordered blue, red, green down the page.
+main() {
+  var div = new DivElement();
+  document.body.elements.add(div);
+
+  // build some DOM elements
+  var bluebox = _colorBox('blue', 40, 20);
+  bluebox.classes.add('foo');
+  var redbox = _colorBox('red', 30, 30);
+  var greenbox = _colorBox('green', 60, 10);
+
+  // assemble DOM
+  var sRoot = new ShadowRoot(div);
+  sRoot.nodes.add(new Element.html('<content select=".foo"></content>'));
+  sRoot.nodes.add(redbox);
+  sRoot.nodes.add(new Element.tag('content'));
+
+  div.nodes.add(bluebox);
+  div.nodes.add(greenbox);
+}
+
+DivElement _colorBox(String color, int width, int height) {
+  var style = ('background-color:$color; '
+      'width:${width}px; height:${height}px;');
+  return new Element.html('<div style="$style"></div>');
+}