This updates the Android build to declare the kernel compile depfile as
an output and its contents as inputs when running with --preview-dart-2
(the default mode).
The 'flutter build aot' command behaves differently depending on whether
it's running in Dart 1 or Dart 2 mode:
* Dart 1: the entrypoint Dart file (typically main.dart) is passed
directly to gen_snapshot, which then emits snapshot.d, whose contents
list the transitive closure of Dart dependencies (input files) for the
snapshot. snapshot.d is a declared output, its contents (plus
gen_snapshot itself) constitute the set of input files to the Gradle
build action.
* Dart 2: then entrypoint Dart file (typically main.dart) is first
compiled with the Dart kernel frontend. This emits kernel_compile.d,
whose contents list the transitive closure of Dart dependencies (input
files) for the kernel 'dill' output file. This 'dill' file is then
passed to gen_snapshot, which emits snapshot.d, whose contents are
empty. As of this change, both snapshot.d and kernel_compile.d are
declared outputs, and their contents (plus gen_snapshot and the
frontend compiler themselves) constitute the set of input files to the
Gradle build action.
This fixes a bug wherein profile/release AOT outputs were not
invalidated due to snapshot.d being empty, and kernel_compile.d being
ignored. This was introduced during recent refactoring of the AOT build
code, wherein the kernel compile and gen_snapshot actions were changed
to emit independent depfiles (previously one stomped -- or failed to --
on the other's output).
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