Closes #539

If the StreamChannel returned by PlatformPlugin.loadChannel() emits an
error through sink.close(), that error is now properly treated as an
error in the load suite for that channel.

Closes #542

This prints debugging information only if the test actually fails.

Fixes error when running with pub.

Failed to precompile test:test:
'package:test/src/frontend/throws_matcher.dart': error: line 17 pos 41: unexpected token ';'

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.

dart-lang/vm_service_client#13 has now landed.

This doesn't fully support the JSON reporter yet.

See #50

Users can pass stayAlive: true to keep them open, but this helps ensure
that they won't accidentally have a bunch of isolates hanging around
between tests.

Closes #109

See #109
Closes #93

This is necessary to make configuration work with Google's internal
build system.

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

Small fixes.

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 .

I think most of these breakages are due to the 1325b694 merge being
slightly busted.

Add debugging support for the JSON reporter.

Also update the pubspec and changelog.

External tools can use this to communicate with the test runner.