Use dart.js in release mode to workaround dartbug.com/19653

pkg/polymer/lib/src/build/polyfill_injector.dart

Index: pkg/polymer/lib/src/build/polyfill_injector.dart
diff --git a/pkg/polymer/lib/src/build/polyfill_injector.dart b/pkg/polymer/lib/src/build/polyfill_injector.dart
index 2f20a526f68148f2024461aa300e09b8498ac66a..baabe102bc3a1a6161248027c8413867af320098 100644
--- a/pkg/polymer/lib/src/build/polyfill_injector.dart
+++ b/pkg/polymer/lib/src/build/polyfill_injector.dart
@@ -78,6 +78,9 @@ class PolyfillInjector extends Transformer with PolymerTransformer {
           if (src.endsWith('.dart')) {
             script.attributes.remove('type');
             script.attributes['src'] = '$src$csp.js';
+            // TODO(sigmund): we shouldn't need 'async' here. Remove this
+            // workaround for dartbug.com/19653.
+            script.attributes['async'] = '';
           }
         }
       } else {