This CL does two things:
* "Reenable" the skipping of the dart:ffi transformation entirely when
the application does not use dart:ffi. This was originally added in
ad596359a6 but was logically disabled in 4558112105 when dart:core
started depending on it. Here we now ignore (real) dart:* libraries
when looking for dependencies of dart:ffi.
* Find the subset of the just compiled libraries that transitively
depend on dart:ffi and just run the dart:ffi transformation on those
libraries.
For dart2js that doesn't use dart:ffi at all we go from:
```
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v pkg/compiler/bin/dart2js.dart | grep -i "ffi"
0:00:09.003428: Transformed ffi natives in 51ms.
0:00:09.203442: Transformed ffi annotations in 199ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v pkg/compiler/bin/dart2js.dart | grep -i "ffi"
0:00:09.098175: Transformed ffi natives in 52ms.
0:00:09.296847: Transformed ffi annotations in 198ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v pkg/compiler/bin/dart2js.dart | grep -i "ffi"
0:00:08.854341: Transformed ffi natives in 57ms.
0:00:09.061804: Transformed ffi annotations in 207ms.
```
to
```
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v pkg/compiler/bin/dart2js.dart | grep -i "ffi"
0:00:09.138651: Skipped ffi transformation in 1ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v pkg/compiler/bin/dart2js.dart | grep -i "ffi"
0:00:09.242590: Skipped ffi transformation in 1ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v pkg/compiler/bin/dart2js.dart | grep -i "ffi"
0:00:09.163885: Skipped ffi transformation in 1ms.
```
When compiling the flutter gallery app that does depend on dart:ffi we go from:
```
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v --target=flutter --platform=/tmp/tmp.6KV5psVGIG/flutter_patched_sdk/platform_strong.dill /tmp/tmp.6KV5psVGIG/gallery/lib/main.dart | grep -i "ffi"
0:00:16.344955: Transformed ffi natives in 78ms.
0:00:16.777218: Transformed ffi annotations in 432ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v --target=flutter --platform=/tmp/tmp.6KV5psVGIG/flutter_patched_sdk/platform_strong.dill /tmp/tmp.6KV5psVGIG/gallery/lib/main.dart | grep -i "ffi"
0:00:15.994674: Transformed ffi natives in 80ms.
0:00:16.472779: Transformed ffi annotations in 478ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v --target=flutter --platform=/tmp/tmp.6KV5psVGIG/flutter_patched_sdk/platform_strong.dill /tmp/tmp.6KV5psVGIG/gallery/lib/main.dart | grep -i "ffi"
0:00:16.027488: Transformed ffi natives in 81ms.
0:00:16.491362: Transformed ffi annotations in 463ms.
```
to
```
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v --target=flutter --platform=/tmp/tmp.6KV5psVGIG/flutter_patched_sdk/platform_strong.dill /tmp/tmp.6KV5psVGIG/gallery/lib/main.dart | grep -i "ffi"
0:00:15.991628: Transformed ffi natives in 30ms.
0:00:16.226564: Transformed ffi annotations in 234ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v --target=flutter --platform=/tmp/tmp.6KV5psVGIG/flutter_patched_sdk/platform_strong.dill /tmp/tmp.6KV5psVGIG/gallery/lib/main.dart | grep -i "ffi"
0:00:16.184984: Transformed ffi natives in 29ms.
0:00:16.404439: Transformed ffi annotations in 219ms.
$ out/ReleaseX64/dart pkg/front_end/tool/_fasta/compile.dart -v --target=flutter --platform=/tmp/tmp.6KV5psVGIG/flutter_patched_sdk/platform_strong.dill /tmp/tmp.6KV5psVGIG/gallery/lib/main.dart | grep -i "ffi"
0:00:15.933513: Transformed ffi natives in 27ms.
0:00:16.145640: Transformed ffi annotations in 212ms.
```
TEST=Assuming tests already exists.
Change-Id: Ie13be1924d3439cb710c782af81e1e42cda2a838
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/228001
Reviewed-by: Johnni Winther <johnniwinther@google.com>
Reviewed-by: Daco Harkes <dacoharkes@google.com>
Commit-Queue: Jens Johansen <jensj@google.com>
Jens Johansen
ae593598f8