Removes the threading from SuperDevListener.

This is causing multi-threading issues with compiler
as it does have un-syncrhonized static state.

I couldn't observe any performance difference above the noise level
while testing this with Hello sample app and GWT source.
In single thread I was getting consistently 19 seconds while I was
getting 18.5 to 19+ seconds with the extra thread. Besides this is
the DevMode launch time so even a few seconds shouldn't matter
much.

Bug: issue 9055
Change-Id: I5384904df9d108ae736b6ce0455b90dfaa6e0c63