* Call weak_ptr.lock(), which returns a null shared_ptr and does not throw
* IsViewInvalid was inverted

Both dart:core and dart:_internal export Symbol. 

This is silently ignored by the VM but some tools (e.g. fasta) report a warning when processing dart:ui sources.

See https://github.com/dart-lang/sdk/issues/30127 for more details.

On emulators, render onscreen (with warnings) if any of the MakeS32 based SkSurface creation calls fail. (#3947)

* Mention Goma for Googlers in CONTRIBUTING.md

* review comments

This updates our compilers, GN and Ninja binaries.

Fixes https://github.com/flutter/flutter/issues/11352

The new `dart_observatory_pub_packages_rev` dependent on the rolled `dart_revision` required the update of the licensing files

Fixes https://github.com/flutter/flutter/issues/11437

Now, frame requests past the pipeline depth will never be deferred due
to back pressure. This backend is only used in the test runner.

* fix (with squashed debug stuff)

* clean up

Clean up log messages a bit.

See https://github.com/flutter/engine/pull/3881

Compute the necessary texture resolution using more accurate scaling
information provided by Mozart scene node metrics events instead of the
device pixel ratio provided by the Mozart view properties (which we
might remove in the future).

This allows us to allocate smaller textures when a Flutter view is
being scaled down.

* Fix Flutter loading from dill files.

* Remove disable of causal async stacks.

* Include mirrors patch files as they are needed for release/profile gen_snapshot

* Free the bytes

* Add FTL_DCHECK

This reverts commit 47349a35.

This reverts commit b48d98e0.

In https://github.com/flutter/engine/pull/3833 the `_flutter.listViews` RPC moved from thread based to lock based synchronization.
The thread based synchronization side effect was used by flutter benchmarks in https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/vmservice.dart#L1223 and
https://github.com/flutter/flutter/blob/master/packages/flutter_tools/lib/src/run_hot.dart#L156 to ensure the completeness of the restart/reload and so correct timing.

A new RPC `_flutter.flushUIThreadTasks` is introduced to allow the flutter benchmarks to reintroduce thread based synchronization.

Related https://github.com/flutter/flutter/issues/11241

Will make debugging layer trees with PhysicalLayers easier.

[fuchsia] Make it easy to flip the whole system between flutter_jit_app and flutter_aot_app. (#3873)