* Drain and ignore

* add changelog and pubspec

* extend timeout

This also makes CompilerPool more general, so it can be used by both
the node and the browser platforms.

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.

Closes #621

Closes #578

Closes #579

Closes #555

Closes #554

We're using a feature that was introduced in
1.6.0 (Disconnector.disconnect's return value).

Closes #556

Closes #539

This prints debugging information only if the test actually fails.

This also adds a (private) AsyncMatcher class that makes it easier to
define asynchronous matchers, and generally improves the formatting of
those matchers.

This reverts commit 35f9d92a.