Fixes needed for the deploy step.

pkg/custom_element/lib/polyfill.dart

Index: pkg/custom_element/lib/polyfill.dart
diff --git a/pkg/custom_element/lib/polyfill.dart b/pkg/custom_element/lib/polyfill.dart
index 0d041aad06eeec1e057811c9239eaeedaef066f5..94c4e8b6cb33b06ef28022c4d652e0114635ddc7 100644
--- a/pkg/custom_element/lib/polyfill.dart
+++ b/pkg/custom_element/lib/polyfill.dart
@@ -21,7 +21,9 @@ Future customElementsReady = () {
   if (_isReady) return new Future.value();
 
   // Not upgraded. Wait for the polyfill to fire the WebComponentsReady event.
-  return document.body.on['WebComponentsReady'].first;
+  // Note: we listen on document (not on document.body) to allow this polyfill
+  // to be loaded in the HEAD element.
+  return document.on['WebComponentsReady'].first;
 }();
 
 // Return true if we are using the polyfill and upgrade is complete, or if we