Group together externally-managed UIDartState.

It's a frequent pattern to enable access to shell resources from the
framework via UIDartState. These handles are passed through several
other objects, where they lay mostly dormant. This refactor just aims
to ease maintenance of this state.

UIDartState is essentially just an easy to extend bus for dropping
off the shell kids in UIDartState, with constructors/operators that
remove the need for providing a full initializer lists in unittests.

This change removes uses of toString transformer from Flutter engine. The transformer is already wired up in Dart SDK: front-end server from Dart SDK supports --delete-tostring-package-uri option. After this change only the integration test is left in the Flutter engine to make sure dart:ui is properly transformed.

dart-lang/sdk#46022

This PR accomplishes many things:
- Unblocks input resampling on Fuchsia
- Fixes 2 frame scheduling issues resulting in wasted work and missed frames
- Removes the now-defunct VsyncRecorder class
- Prepares the VsyncWaiter class for the Embedder API by moving all logic out of it
- Adds hundreds of lines of unittests for Fuchsia scheduling logic to validate the performance and correctness benefits

https://skia.googlesource.com/skia.git/+log/1f193df9b393..edb7aa70fcf3

2021-05-20 bsalomon@google.com Revert "Add support for do-while loops in PipelineStage."

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/skia-flutter-autoroll
Please CC bsalomon@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md

https://dart.googlesource.com/sdk.git/+log/67be110b5ba8..3a8c551ec08e

2021-05-19 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-128.0.dev
2021-05-19 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-127.0.dev
2021-05-19 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-126.0.dev
2021-05-18 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-125.0.dev
2021-05-18 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-124.0.dev
2021-05-18 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-123.0.dev
2021-05-18 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-122.0.dev
2021-05-17 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-121.0.dev
2021-05-17 dart-luci-ci-builder@dart-ci.iam.gserviceaccount.com Version 2.14.0-120.0.dev

If this roll has caused a breakage, revert this CL and stop the roller
using the controls here:
https://autoroll.skia.org/r/dart-sdk-flutter-engine
Please CC dart-vm-team@google.com on the revert to ensure that a human
is aware of the problem.

To report a problem with the AutoRoller itself, please file a bug:
https://bugs.chromium.org/p/skia/issues/entry?template=Autoroller+Bug

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+doc/master/autoroll/README.md