Build shadow_dom package in dart/pkg

pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill-before.js

+/*
+ * Copyright 2013 The Polymer Authors. All rights reserved.
+ * Use of this source code is governed by a BSD-style
+ * license that can be found in the LICENSE file.
+ */
+(function() {
+  // TODO(jmesserly): fix dart:html to use unprefixed name

[Jennifer Messerly, 2013/08/15 21:01:37]

I wonder if this is fixed now. I think it might be. maybe we can delete this
whole file now :)

+  if (Element.prototype.webkitCreateShadowRoot) {
+    Element.prototype.webkitCreateShadowRoot = function() {
+      return window.ShadowDOMPolyfill.wrapIfNeeded(this).createShadowRoot();
+    };
+  }
+})();