* 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.
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