Add token to stderr as well as stdout for batch processing.

Add token to stderr as well as stdout for batch processing.

This makes it easy to tell when a test is completed and we have all
the output. stderr and stdout are independent file descriptors so
this is the only reliable way to know that you have flushed both.

Committed: https://code.google.com/p/dart/source/detail?r=4782

[Mads Ager (google), 2012-03-01 08:41:16]

I reverted the previous change to fix this quickly. There were some cleanup
changes that also got reverted. Feel free to add that back.

[Jennifer Messerly, 2012-03-01 09:07:10]

lgtm. Nice to be waiting for stderr too :)