Fixed bug in browser_controller.dart

Fixed bug in browser_controller.dart

If a browser crashed on it's own, then the driving page running in it will not
post a result. So the test will eventually time out. The testing server will
then call 'browser.close()' and after the browser was closed, it will start a
new one.

Previously the 'browser.close()' future did not complete in this scenario, since
the '_browserClosed' callback was not set.

Note that with the new browser controller, browser crashes will currently be
reported as timeouts.

R=ricow@google.com

Committed: https://code.google.com/p/dart/source/detail?r=23951

[kustermann, 2013-06-12 22:33:22]

There may still be more issues.

[ricow1, 2013-06-13 06:29:45]

LGTM
We should add a callback future that actually completes when the browser closes
to catch crashes

[kustermann, 2013-06-13 08:08:41]

Committed patchset #1 manually as r23951 (presubmit successful).