Integrating fix for IE9 custom elements upgrade

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 510dc2cea79cb52e089d49cf6129125b59c2991c..11ae6246b304eb3c1c91e4e7acb890099ebaa041 100644
--- a/pkg/custom_element/lib/custom-elements.debug.js
+++ b/pkg/custom_element/lib/custom-elements.debug.js
@@ -1176,7 +1176,7 @@ if (useNative) {
     // HTMLElement.prototype, so we add a test
     // the idea is to avoid mixing in native prototypes, so adding
     // the second test is WLOG
-    while (p !== inNative && p !== HTMLUnknownElement.prototype) {
+    while (p && p !== inNative && p !== HTMLUnknownElement.prototype) {
       var keys = Object.getOwnPropertyNames(p);
       for (var i=0, k; k=keys[i]; i++) {
         if (!used[k]) {