* Reland fuchsia external view embedder will be shared with platform view

This reverts commit 9b752790.

* wait for the external view embedder to be initialized before creating
shell

This reverts commit 1bc025d6.

Snapshots compiled with sound null-safety enabled require changes to the way in
which isolates are launched. Specifically, the `Dart_IsolateFlags::null_safety`
field needs to be known upfront. The value of this field can only be determined
once the kernel snapshot is available. This poses a problem in the engine
because the engine used to launch the isolate at shell initialization and only
need the kernel mappings later at isolate launch (when transitioning the root
isolate to the `DartIsolate::Phase::Running` phase). This patch delays launch of
the isolate on the UI task runner till a kernel mapping is available. The side
effects of this delay (callers no longer having access to the non-running
isolate handle) have been addressed in this patch. The DartIsolate API has also
been amended to hide the method that could return a non-running isolate to the
caller.  Instead, it has been replaced with a method that requires a valid
isolate configuration that returns a running root isolate. The isolate will be
launched by asking the isolate configuration for its null-safety
characteristics.

A side effect of enabling null-safety is that Dart APIs that work with legacy
types will now terminate the process if used with an isolate that has sound
null-safety enabled. These APIs may no longer be used in the engine. This
primarily affects the Dart Convertors in Tonic that convert certain C++ objects
into the Dart counterparts. All known Dart Converters have been updated to
convert C++ objects to non-nullable Dart types inferred using type traits of the
corresponding C++ object. The few spots in the engine that used the old Dart
APIs directly have been manually updated. To ensure that no usage of the legacy
APIs remain in the engine (as these would cause runtime process terminations),
the legacy APIs were prefixed with the `DART_LEGACY_API` macro and the macro
defined to `[[deprecated]]` in all engine translation units. While the engine
now primarily works with non-nullable Dart types, callers can still use
`Dart_TypeToNonNullableType` to acquire nullable types for use directly or with
Tonic. One use case that is not addressed with the Tonic Dart Convertors is the
creation of non-nullable lists of nullable types. This hasn’t come up so far in
the engine.

A minor related change is reworking tonic to define a single library target.
This allows the various tonic subsystems to depend on one another. Primarily,
this is used to make the Dart convertors use the logging utilities. This now
allows errors to be more descriptive as the presence of error handles is caught
(and logged) earlier.

Fixes https://github.com/flutter/flutter/issues/59879

Reland #19396 with a fix for improper scale that was affecting internal tests

Tested: Ran all unittests, ran internal tests, and ran workstation on Fuchsia
BUG: 53062, 53063

This improves raster cache control from apps and enables
GPU acceleration for some offscreen workloads that would
fallback to software.

Test: PlatformViewTests.GetGrContextTest
Co-authored-by: NDavid Reveman <reveman@google.com>

This reverts commit 12a37478.

The fuchsia code around metrics and sizing was just sending this
information through a side-channel, when the engine already had the
information available. So, delete all of it to make future CLs simpler.

Additionally, the SceneUpdateContext has many unneccesary dependencies
re: metrics and PaintTasks. Break those to make future CLs simpler.

Tested: Ran all unittests and ran workstation on Fuchsia
BUG: 53062, 53063

* Add PlatformView support for Fuchsia

This change allows embedding views provided by fuchsia components into
a flutter app running on Fuchsia. This conforms to Flutters idiomatic
approach to composite PlatformView alongside other rendered layers.

This uses the `view embedder` infrastructure to allow
`PlatformViewLayer`
to hold fuchsia views. This is meant to eventually supplant the legacy
`SceneHost` and `ChildViewLayer` mechanism to embed fuchsia `ChildView`.

To see how this will get used check out:
https://fuchsia-review.googlesource.com/c/experiences/+/398536/6/examples/hello_experiences/lib/fuchsia_view.dart

Includes unittests for platform_view.cc.

Note: This change has no impact on the legacy code to embed fuchsia
views.

* Rename OnCreateViewMethodCall to OnCreateView

Same for OnDestroyViewMethodCall to OnDestroyView
Co-authored-by: NSanjay Chouksey <sanjayc@google.com>

The last commit was a bad push -- this one compiles.

Tested: Manual test via workstation product
BUG: 50875

This reverts commit 40043b5f.

Tested: Manual test via workstation product
BUG: 50875

In order to better support different products on Fuchsia, we should
change performance-sensitive attributes based on config files passed in.
This change does so for `vsync_offset`.

Revert "Migrate flutter_runner from flutter_runner::{Thread,Loop} to fml::{Thread,MessageLoop} (#15118)" (#16277)

This reverts commit 41e8ed07.

fixes https://github.com/flutter/flutter/issues/47178

Revert "Migrate flutter_runner from flutter_runner::{Thread,Loop} to fml::{Thread,MessageLoop} (#15118)" (#15903)

This reverts commit a50f1ef5.

The FIDL service `fuchsia.intl.PropertyProvider` is a service that
the flutter runner can use to obtain information on system preferred
locales.

This change sends a platform message "setLocale" on the channel
"flutter/localization", based on the values provided by the above
mentioned FIDL service.

Credit: most of this was initially written by @kpozin; I ported it
to out-of-tree flutter engine.

Tested:

1. Compile and publish the unit tests package as shown in
   the script below.
2. In a Fuchsia repository (pointed to by `$FUCHSIA_DIR`), run
   `fx serve`
3. `fx shell run fuchsia-pkg://fuchsia.com/flutter_runner_tests#meta/flutter_runner_tests.cmx`

The script used to update the unit tests.

```bash

set -x

FLUTTER_ENGINE_DIR="${FLUTTER_ENGINE_DIR:-$HOME/fx/flutter/engine/src}"
readonly OUT_DIR="${FLUTTER_ENGINE_DIR}/out"

(
  cd ${FLUTTER_ENGINE_DIR}
  ./flutter/tools/gn --fuchsia --fuchsia-cpu x64 --unoptimized
  ninja -j 100 -C "${OUT_DIR}/fuchsia_debug_unopt_x64"
  cp "${OUT_DIR}/compile_commands.json" "${FLUTTER_ENGINE_DIR}"

  echo "Publishing the tests package"
  "${FLUTTER_ENGINE_DIR}/fuchsia/sdk/linux/tools/pm" publish \
    -a -r $FUCHSIA_DIR/out/release/amber-files \
    -f "${FLUTTER_ENGINE_DIR}/out/fuchsia_debug_unopt_x64/flutter_runner_tests-0.far"
)
```

Build rules still reference creating share snapshot data and instructions. This makes the engine to always pass them as empty to the dart vm. To be followed up with a change to alter the build rules to stop referencing the shared snapshots.

This is not being used currently and the fact that the runner will be built outside of the flutter tree means that the apps will not have much to gain via shared snapshots. The rationale behind this change is to partially make migrating the runner out of topaz tree easier.

Change-Id: Ibc4dd6a298d65082416af753522f5a17c88a750a

Expose scenic's ability to toggle wireframe debug
rendering of view bounds in flutter_runner. This is done
by registering a new function on the platform_views channel
with the PlatformView.

Note: Unittests have not been enabled, will enable once we
have sufficient infra.

SCN-1351 #done

Change-Id: Id4c8ef65cc39a967087d7fa6c9f595da8cfe5f01

This makes room for the Dart runner at //flutter/shell/platform/fuchsia/dart.

Build rules and include paths have been updated as necessary.
No other functional change.