Yes, if you have regular hand-written JS, and let's say you run UglifyJS, YUI Minifier, or Closure Compiler, to generate compact, optimized JS, with source maps, the debugger will show the original JS source.
The demo I showed also is using SuperDevMode for GWT, which dramatically speeds up the compile. Notice, I compile a project (the Mail sample) of several hundred Java source files in less than 2 seconds.