See https://github.com/dart-lang/sdk/issues/28463, which I think is a regression. This also fixes the regression introduced by https://codereview.chromium.org/2559773002 whereby we were no longer checking any of the lints.
We now produce a more reasonable error message when we're missing the flutter_test dependency in a test. Also, remove the flutter_tools stack traces when the engine dies. Fixes #6187