This change re-introduces skipping snapshot builds if input sources (and
outputs) have not changed since the last snapshot build, with a bugfix
to include the entry-point source in the checksum used to check whether
rebuild can be skipped. This ensures that the following sequence
invalidates the cached build, resulting in two snapshot builds:
flutter build ios lib/foo.dart
flutter build ios lib/bar.dart
This reverts commit 3d5afb5a81052b7d55661549320d6d43f893f448.