fix(dart/transform): Remove malfunctioning zone error handler

Remove `onError` zone callback which is consuming exceptions thrown by
the `Transformer`s and can cause `pub` to become unresponsive.

Closes #3368

modules/angular2/src/transform/reflection_remover/transformer.dart

@@ -52,6 +52,6 @@ class ReflectionRemover extends Transformer {
           mirrorMode: mirrorMode, writeStaticInit: writeStaticInit);
       transform.addOutput(
           new Asset.fromString(transform.primaryInput.id, transformedCode));
-    }, errorMessage: 'Removing reflection failed.');
+    });
   }
 }