fix an issue w/ event listeners in M35

This fixes (or at least works around) an issue where event objects are not properly proxied in M35 and later. This issue does not show up in M33, and I don't think in M34. This issue would cause any code trying to listen for network, storage, window, ... events to throw when compiled to JS on M35 or later.

@financeCoding 

https://github.com/dart-lang/spark/issues/1268