This reverts commit d48de7a3.

From build log:
```
../../flutter/shell/platform/android/io/flutter/view/FlutterView.java:95: error: cannot find symbol
    private final LocalizationChannel localizationChannel;
                  ^
  symbol:   class LocalizationChannel
  location: class FlutterView
../../flutter/shell/platform/android/io/flutter/view/FlutterView.java:172: error: cannot find symbol
        localizationChannel = new LocalizationChannel(dartExecutor);
                                  ^
  symbol:   class LocalizationChannel
  location: class FlutterView
2 errors
```

This reverts commit 256db4bc.

The shell was already designed to cleanly shut down the VM but it couldnt
earlier as |Dart_Initialize| could never be called after a |Dart_Cleanup|. This
meant that shutting down an engine instance could not shut down the VM to save
memory because newly created engines in the process after that point couldn't
restart the VM. There can only be one VM running in a process at a time.

This patch separate the previous DartVM object into one that references a
running instance of the DartVM and a set of immutable dependencies that
components can reference even as the VM is shutting down.

Unit tests have been added to assert that non-overlapping engine launches use
difference VM instances.

On Android we were never deleting the textures allocated for
the texture registry, which resulted in a memory leak, see:
https://github.com/flutter/flutter/issues/24145

Added `FlutterPluginRegistrantCallback` typedef and optional `setPluginRegistrantCallback` static method in `FlutterPlugin`. `setPluginRegistrantCallback` is used to set a callback defined in AppDelegate.m which registers some subset of plugins with a `FlutterPluginRegistry` (for example, a headless FlutterEngine). This allows for plugins which utilize background execution functionality to enable the use of other plugins on the background isolate. This puts iOS on par with Android for background execution.

Along with updated examples and Medium posts, this fixes [flutter/flutter#21925](https://github.com/flutter/flutter/issues/21925).

Also extend the embedder unit tests to cover AOT execution

(Authored by @nathanrogersgoogle )

Fixes https://github.com/flutter/flutter/issues/27641

Provide public api to allow FlutterEngine related context  to be destoryed.

If a frame is scheduled by the time the delayed animator notify idle
task runs, then we should not call |NotifyIdle|.

Also, add a trace event in the task, to make it more clear in traces
when a notify idle call is coming from here (as opposed to
|Animator::AwaitVSync|).

In 93452747 (#7642), //shell/platform/darwin:framework_shared was added
without the root flutter config, causing a build breakage in Fuchsia,
where flutter sits under //third_party/flutter as opposed to at the
root. The flutter root config is required to set -I../../third_party as
an include path.

Avoids potential confusion due to the iOS framework already being called
Flutter.framework.

* Dynamic patching support for AOT mode.

* Cleanup.

Allows embedders to specify a callback to be invoked in isolate scope
once root isolate has been created and marked runnable.

As an example of where this is useful, embedder unit test fixtures may
want to include Dart functions backed by a native implementation. On
isolate creation, this patch allows the unit test author to call
Dart_SetNativeResolver in root isolate scope.

This reverts commit 093b2fea.

For https://github.com/flutter/flutter/issues/24058, We now only get a
"Program linking failed" error message on Windows Android simulators
without crahsing. Skia seems to have handled the linking failure in
https://skia-review.googlesource.com/c/skia/+/180372 by rebuilding the
program.

Merges the current flutter-desktop-embedding macOS framework into the
engine. Notable changes:
- All channel/codec related code is eliminated in favor of using the
  existing iOS implementations.
- All .m files renamed to .mm for consistency with the iOS code.
  - Some minor code changes to fix new warnings in Objective-C++ mode.
- License headers, basic format (e.g., clang-format changes) updated to
  use repo style.
- Xcode project is not included; instead adds GN build rules to create
  an integrated framework that combines what was the FDE library with
  what is present in FlutterEmbedder.framework.

Other changes are left as follow-ups, including:
- Moving shared code out of ios/ into common/.
- Potentially improving sharing between iOS and macOS BUILD.gn.
- Class renaming; the FLE prefix will be eliminated, but that API
  surface isn't stable yet, so that can be changed later.

- Moved versions from shell to common
- versions singleton contains all the required versions.

Document make_resource_current on FlutterOpenGLRendererConfig and warn if the callback is not set. (#7648)

Prior to this the Android embedder code would extract the icudtl.dat asset out
of the APK and write it to local disk during the first startup of the app.

This change will make that work unnecessary and eliminate the risk of ICU
failures due to errors in the extraction process.

FlutterResult is also the name of a class in the Objective-C API
surface, which is problematic when building a framework that contains
both (such as a macOS implementation of the Flutter framework).

ForwardingGestureRecognizer was previously accepting the gesture when the touch sequence ended, this was causing some other gesture recognizers to not accept the gestures (e.g tapping on the my location button in a Google Map didn't work).