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..e994f00d61a0bb88cb9c59bb79f9e0dada7a0c82 100644
--- a/pkg/polymer/lib/src/build/polyfill_injector.dart
+++ b/pkg/polymer/lib/src/build/polyfill_injector.dart
@@ -78,6 +78,8 @@ class PolyfillInjector extends Transformer with PolymerTransformer {
           if (src.endsWith('.dart')) {
             script.attributes.remove('type');
             script.attributes['src'] = '$src$csp.js';
+            // TODO(sigmund): remove async (workaround for dartbug.com/19653).

[Jennifer Messerly, 2014/06/26 19:17:54]

at first glance I was confused by this comment because it said "remove" but
we're actually adding it :)
Maybe:

TODO: for now add 'async' as a workaround for dartbug.com/19653. 

[Siggi Cherem (dart-lang), 2014/06/26 20:00:52]

ha ha, I normally write TODOs as the action that needs to be done to fix the
code, rather than what we are currently doing, but I can see now that "remove
async" was ambiguous :-)

I rephrased it a bit.

+            script.attributes['async'] = '';
           }
         }
       } else {