cb00aac583
* Allow specifying both Dart and non-Dart fixtures in engine unittests. This fixes numerous issues in the way in which fixtures were managed in the engine unit-tests. * Instead of only being able to specify Dart fixtures, unit-tests may specify non-Dart fixtures as well. These are simply copied over to the fixtures directory known to the unit-test at runtime. * An issue where numerous Dart files could be given to the kernel snapshotter has been addressed. It was anticipated that such a (legal) invocation to the kernel snapshotter would produce a snapshot with the contents of all the Dart files added to the root library. This is incorrect and the behavior in this case is undefined. * Dart files referenced by the main Dart file are correctly tracked via a depfile. * The snapshotter arguments have been cleaned up to get rid of unused arguments (`—strong`) and the use of the VM product mode argument has been corrected to no longer depend on the Flutter product mode.