* Use snapshot's .d file as source inputs in Gradle build.

If we don't yet have a .d file (first build), fall back to using the
.dart files in the current directory. This enables us to detect changes
in dependent source files (Flutter framework, packages outside the
source directory, etc.), and re-generate the snapshots as needed.

Unfortunately, Gradle requires knowing the source files before executing
the task, and can't update them after building, so Gradle considers the
second build to be out-of-date (because it has more input files than the
first build). Sub-sequent builds have the correct dependency
information, and will be skipped if the source files haven't changed.

Also added a dependency on gen_snapshot. The snapshot ABI isn't stable,
so we need to re-generate the snapshots when we roll the Dart SDK
dependency.

Fixes #8315
Fixes #8687
Fixes #8607