Run tear-downs in the same error zone as tests.

We used to create a new error zone for each call to
waitForOutstandingCallbacks(), which meant that asynchronous errors
couldn't be safely passed around within a single test. Now we have a
single error handler that manually checks the zone in which the error
was thrown.

Enable associated lint
Verified the new lower bound (v0.26.4 from Dec 7, 2015) works

Stop using scheduled_test.

Enable browser tests on Travis.

Closes #621

Closes #578

Closes #579