This assumes a fix to https://github.com/dart-lang/test/issues/542

The timeout added in this patch is a workaround for https://github.com/dart-lang/coverage/issues/165

* Small Flutter strong mode cleanup fixes.

These are cases where strong mode down cast composite errors
generally indicated cases that would performance or correctness
issues if Flutter code was run in a strong mode VM.

* Fix Command API so that it is always in terms of Map<String,String>.

* Fix typedef

...now that we have generic methods, their types need to be specified too.

Also, defer to test package for throttling (this will require a test
package update as well).

Also, add a lot more instrumentation to --verbose mode for tests, and
fix minor trivial things here and there, and add error handling in
more places.

Also, refactor how coverage works to be simpler and not use statics.

Previously, it was possible for the test harness to bail
and the test runner to complete before the platform plugin
triggered the collection of coverage data. This fixes the
race condition such that the pending coverage collection
task is recorded immediately after starting the process.

This ensures that accidental usages of dart:io's
file API don't creep in over time.

Previously these widget had zero test coverage. Also, improve the
coverage tooling Mac.

Using relative paths makes it easier to share lcov.info files across machines.

This patch causes us to upload a coverage report to coveralls.io.

We need https://github.com/dart-lang/coverage/issues/100 to be fixed before
this will be useful.

Fixes #2342