This can never fire before the load suite timeout. Now that that
timeout actually works, it's redundant.

See #390

This makes it easier to add platforms that extend existing platforms
without running into serialization headaches. It also cleans up the
Declarer API and means we're sending less redundant data between
isolates.

This ensures that the remote listener has access to any platforms that
are dynamically loaded in the test runner, so they can be used in
platform selectors.

See #99
See #391

* extend timeout

Dartfmt *.dart

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 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

This tracks configuration that makes sense to apply to test suites, as
opposed to Configuration which applies to an entire run and Metadata
which applies to individual tests. This is intended to make it easier
for load separate configurations for separate targets in a live test
runner.

Don't capture stack traces by default.

Stack traces are now only captured for the JSON reporter. Capturing
traces was slow enough to be a serious problem for test suites with many
tests.

Closes #457

Closes #420

Review URL: https://codereview.chromium.org//2066113002 .

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1960503002 .

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1732773003 .

Also, we now determine whether the user is debugging a given browser in
BrowserManager rather than individual suites.

R=kevmoo@google.com

Review URL: https://codereview.chromium.org//1704773002 .