Including the module name in the magic sourceURL comment in code generated by the iframe linker.
This gives a bit more context in the browser debugger, and is especially important when including
multiple GWT applications on a page, since otherwise they will all be named "0.js" and the
browser will arbitrarily choose one of them to show.

Change-Id: I4044b777c29b058584efd756a7987244d9f7a158