## Description
As the related issue refer, the application may be doing too much work on its main thread even in a simple hello_world demo.
That is because the creation of `Engine` on the ui thread takes a noticeable time, and it is blocking the platform thread in order to run `Shell::Setup` synchronously.
The cost of `Engine`'s constructor is mainly about the creating of root isolate. Actually, there used to be another time-consuming process, the default font manager setup, which was resolved by https://github.com/flutter/engine/pull/18225. 
Similar to https://github.com/flutter/engine/pull/18225, this pr move the creation of root isolate out from creating `Engine`. After this action, the main thread blocking is quite an acceptable slice.

## Related Issues
https://github.com/flutter/flutter/issues/40563 could be resolved by this pr.

This caused over-aggressive GCs, which vastly increased CPU usage benchmarks.

* Revert "fix build (#20644)"

This reverts commit b59793ee.

* Revert "Hint freed (#19842)"

This reverts commit 3930ac1b.

* Hint the VM when a layer or picture goes out of scope

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 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

This reverts commit 6ea69a0d.

On top of the revert, it reverted a commit in the PR:
https://github.com/flutter/engine/pull/14024

This reverts commit ea67e5b0b930ebf552bc7dbd678a35ee6a129c39.

This reverts commit d117ac97.

* Remove erroneous ChildView opacity

* Wire frame metrics through contexts

* Maintain layer stack inside of SceneBuilder

* Remove EnsureSingleChild

* Centralize system-composite and elevation logic

* Wire up OpacityLayer to Scenic

Moved our code to passing functions by const ref

This reverts commit 8ebb3184.

This reverts commit 1bfb928e.

* Reland "Added new lifecycle enum (#11913)"

This reverts commit 02a47900.

Put `Picture.toImage` back on the GPU thread.  Left the unit tests intact.

Remove dead shared snapshot arguments to Dart_CreateIsolateGroup.

6a65ea9cad4b [vm] Remove shared snapshot and reused instructions features.
db8370e36147 [gardening] Fix frontend-server dartdevc windows test.
4601bd7bffea Modified supertype check error message to be more descriptive.
0449905e2de6 [CFE] Add a serialization-and-unserialization step to strong test
c8b903c2f94f Update CHANGELOG.md
2a12a13d9684 [Test] Skips emit_aot_size_info_flag_test on crossword.
b26127fe01a5 [cfe] Add reachability test skeleton

This reverts commit 9dd585c4.

Obtaining the SkiaUnrefQueue through the IOManager is unsafe because
UIDartState has a weak pointer to the IOManager that can not be dereferenced
on the UI thread.

This reverts commit 6c2381da and applies iOS fixes.

This reverts commit e53d10c3.

The contents rendered into the backing stores are already correctly scaled.
The initial implementation assumed this also held true for the metrics obtained
via embedded view parameters.

Fixes b/142699417

Since this is currently only meant to be used by the embedding internally, the setter in Objective-C is only exposed via the FlutterDartProject private class extension. Unit tests have been added to the shell_unittests harness.

Fixes https://github.com/flutter/flutter/issues/37641

This reverts commit c2879cae.

Additionally, we fix https://github.com/flutter/flutter/issues/40863 by adding a secondary VSYNC callback.

Unit tests are updated to provide VSYNC mocking and check the fix of https://github.com/flutter/flutter/issues/40863.

The root cause of having https://github.com/flutter/flutter/issues/40863 is the false assumption that each input event must trigger a new frame. That was true in the framework PR https://github.com/flutter/flutter/pull/36616 because the input events there are all scrolling move events. When the PR was ported to the engine, we can no longer distinguish different types of events, and tap events may no longer trigger a new frame.

Therefore, this PR directly hooks into the `VsyncWaiter` and uses its (newly added) secondary callback to dispatch the pending input event.

This reverts commit fcc4ab32.

Fixes https://github.com/flutter/flutter/issues/41394 and other
related correctness issues.

TBR: @arbreng @jason-simmons @mehmetf 

On Fuchsia, add a build flag for compositing OpacityLayers using the system
compositor vs Skia, which exposes a fastpath for opacity via Scenic.
This will only work under certain circumstances, in particular nested
OpacityLayers will not render correctly!

On Fuchsia, add a build flag for compositing PhysicalShapeLayers using
the system compositor vs Skia. Set to off by default, which restores
performant shadows on Fuchsia.

Remove the opacity exposed from ChildView, as that was added mistakenly.

Finally, we centralize the logic for switching between the
system-composited and in-process-composited paths inside of
ContainerLayer. We also centralize the logic for computing elevation
there. This allows the removal of many OS_FUCHSIA-specific code-paths.

Test: Ran workstation on Fuchsia; benchmarked before and after
Bug: 23711
Bug: 24163

* Fix broken tests

This reverts commit aac33d1b.

Reason: flutter/flutter#40863

TBR: chinmaygarde, iskakaushik

Additionally, we now use the engine directly as a delegate instead of storing potentially dead runtime_controller.

Unit tests have been updated to include an engine restart check which would fail before the fix.

This fixes https://github.com/flutter/flutter/issues/40303