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.

This doesn't fully support the JSON reporter yet.

See #50

See #109
Closes #93

This worked with the old expectAsync(), but was broken when the new
functions were introduced. It was causing an analysis error in the
tests.

This adds `spawnHybridUri()` and `spawnHybridCode()` methods that spawn
VM isolates even when called from the browser. It still needs some
additional work to close the isolates when the test ends by default.

See #109

BUG=
R=nweiz@google.com

Review URL: https://codereview.chromium.org//2557183002 .

Dartium wasn't emitting any standard IO on Windows, which caused us to
stall out waiting for the Observatory URL in debug mode.

See dart-lang/sdk#28034
Closes #425

Closes #503

Review URL: https://codereview.chromium.org//2526223002 .

Fixes #436.

R=nweiz@google.com

Review URL: https://codereview.chromium.org//2515303002 .

Also update the pubspec and changelog.