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.

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

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).

As of Dart 2, running from Dart source is no longer supported.  Dart
code should now be compiled to kernel form and will be loaded by from
kernel.blob in the assets directory. We retain the struct members for ABI
stability. package_path is also not required since kernel blobs are
self-contained.

This updates the embedder API but introduces no breaking ABI/API
changes.

* Allow embedder clients to provide a resource context. Closes flutter/flutter#14263.

This updated the native embedder API in a non-ABI breaking way.

* The Flutter engine will be shipped as a shared library.
* The engine is renderer and window toolkit agnostic.
* The simple public C API is described in embedder.h.
* ABI breaking changes will be indicated by changing the FLUTTER_ENGINE_VERSION.
* A simple GLFW based example of this API is available at https://gist.github.com/chinmaygarde/8abf44921f7d87f6da7bf026267c4792