Build shadow_dom package in dart/pkg

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

Index: pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill-before.js
diff --git a/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill-before.js b/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill-before.js
new file mode 100644
index 0000000000000000000000000000000000000000..fe15015bdc9fb3188437619a398b0fc3529220e8
--- /dev/null
+++ b/pkg/shadow_dom/lib/src/platform/patches-shadowdom-polyfill-before.js
@@ -0,0 +1,13 @@
+/*
+ * 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
+  if (Element.prototype.webkitCreateShadowRoot) {
+    Element.prototype.webkitCreateShadowRoot = function() {
+      return window.ShadowDOMPolyfill.wrapIfNeeded(this).createShadowRoot();
+    };
+  }
+})();