Quick fix for sendMessage exception

This is what I am using locally to work around https://github.com/dart-gde/chrome.dart/issues/202
I definitely don't think this is a good fix, it just got me rolling again.  The exception is from within the Chrome Extension code, so I'm not really sure what is going wrong.  The exception message makes it sound like it is trying to send a message to a different extension:
```
Uncaught Error: Could not establish connection. Receiving end does not exist.
```