addTearDown() now adds a tearDownAll() rather than adding a plain
tearDown() when called from within setUpAll() (or tearDownAll()). This
is technically a behavioral change, but the old behavior was so
useless I doubt anyone was relying on it.

Closes #712

Use Stdout.terminalColumns for line length

Fall back on a 200-character default, which is less likely to obscure
test names when printing to a non-terminal destination.

Closes #86 

This makes it possible to use the same spawnHybridUri() call from
multiple test files that are in different places in the filesystem.

See #391

Closes #688

* Adapt for strong-mode changes to zone.

* Use `runGuarded` instead of `runUnaryGuarded`.

* Increment version number.

* Add Changelog entry.

* 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