A routine cause of Windows devicelab failures at this point is that the flutter directory can't be deleted after tests because a flutter (dart.exe) process is still running that should have finished.

We could easily add a script to kill the remaining processes on Windows, but ideally we wouldn't be leaking the process to begin with.

We should:

• update our tests on the devicelab to make sure we're not leaking any processes we control at the end. If we do, the test that leaked should fail rather than the next test that can't clean up.
• Fix whatever bugs in the tool and/or dart that are causing this leak