added the ability to test the chrome+dart2js version of the app (as well...

So this CL:

 * does a tiny bit of cleanup, to get things compiling happily
 * changes tool/test_runner.dart to add two flags, `--dartium` and `--chrome`

--dartium is shorthand for: run the app/ version of the application using Dartium
--chrome is shorthand for: run the build/deploy-test-out/web version of the application using Chrome

I won't enable the --chrome on the build bot yet, as the build/deploy-test-out version of the app doesn't currently run, due to various and inscrutable polymer issues.

@ussuri @dinhviethoa