Avoid spurious descheduling when posting message loop tasks.

[rmacnak-google]

Closes dart-lang/sdk#29971

[chinmaygarde]

If the new target time is less than the last wake, the loop will wake up too late.

I am working on a similar patch that keeps track of the wake times.

[rmacnak-google]

PTAL. Addressed the case of a new task being due sooner. Releasing the mutex before calling WakeUp.

[Hixie]

This seems eminently unit-testable, any chance we could test it?

[chinmaygarde]

The logic reads fine to me. We already have unit tests setup for message loops. https://github.com/flutter/engine/blob/c920002508e9ff60a83a30087c8634af9462f9f0/fml/message_loop_unittests.cc. Can we add tests that assert the number of times a wake was called as well as asserting that there aren't any spurious wakes. I am more concerned about the latter case.

[rmacnak-google]

PTAL.

• Added assert again spurious wakeup.
• Added fml_unittests to run_tests.sh
• Added unit test for needing a sooner wakeup.