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 3d5afb5a81.
Previously, the snapshot file was recomputed on every build. We now
record checksums for all snapshot inputs (which are catalogued in the
snapshot dependencies file output alongside the snapshot) and only
rebuild if the checksum for any input file has changed.
Relocates two tests alongside other related tests:
* moved code_signing_test.dart alongside other lib/src/ios tests
* moved terminal_test.dart alongside other lib/src/base tests
