* 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
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.
