Ability to run tests in parallel with builds

[matanlurey]

More of a long-term/tracking issue, but in a repository with 100s of tests, it would be nice if the tests that have finished building could start running while other tests are still being built.

[jakemac53]

Unfortunately because of how we do the builds in phases (generally) I don't think you would end up getting a ton of value out of this.

For instance for a dart2js test - they all have a post process step which runs after dart2js (to extract the archive if present). Today that won't start until all dart2js invocations are done.

For dartdevc tests, most of the time is spent compiling shared dependencies so all the actual entrypoints likely end up completing around the same time.

We do want to eventually look into a less strict phase-based build which might help the dart2js case at least.