update our CustomElement+Shadow DOM patch

pkg/custom_element/lib/custom-elements.debug.js

Index: pkg/custom_element/lib/custom-elements.debug.js
diff --git a/pkg/custom_element/lib/custom-elements.debug.js b/pkg/custom_element/lib/custom-elements.debug.js
index 7ff04c01d1a5b749354041f53a8fe9795e13e14d..fff3b8aceaaf449d3a2b62625ffbc22a12df72e3 100644
--- a/pkg/custom_element/lib/custom-elements.debug.js
+++ b/pkg/custom_element/lib/custom-elements.debug.js
@@ -1442,13 +1442,14 @@ if (document.readyState === 'complete' || scope.flags.eager) {
 
 (function() {
 // Patch to allow custom element and shadow dom to work together, from:
-// https://github.com/Polymer/platform/blob/master/src/patches-shadowdom-polyfill.js
+// https://github.com/Polymer/platform-dev/blob/60ece8c323c5d9325cbfdfd6e8cd180d4f38a3bc/src/patches-shadowdom-polyfill.js
 // include .host reference
 if (HTMLElement.prototype.createShadowRoot) {
   var originalCreateShadowRoot = HTMLElement.prototype.createShadowRoot;
   HTMLElement.prototype.createShadowRoot = function() {
     var root = originalCreateShadowRoot.call(this);
     root.host = this;
+    CustomElements.watchShadow(this);
     return root;
   }
 }