* Namespaces have been updated to reflect the move from //flutter/sky/shell to //flutter/shell.
* shell/BUILD.gn file has been split into smaller GN files for each subcomponent of the shell (common, GPU, diagnostic, testing).
* GN dependencies have been rewritten to stop exposing common shell dependencies as public. Duplicates have also been removed.
* GPU subcomponent has been updated make it more suitable for Vulkan integration.
* The GLFW backend has been resurrected.

Replicates iOS behavior of placing trace files in the documents directory.

This patch hides the base::SingleThreadedTaskRunner, which pulls a big
switchover to ftl::WeakPtr and std::function.

Remove dependencies on ui, third_party/mesa and mojo/convertors. Provide simple alternatives. (#2783)

Previously we enabled accessibility unconditionally on iOS, which is more
expensive than necessary. We still enable it unconditionally on the simulator
because there's no API for determining whether accessibility is needed on the
simulator.

1) Add ability to specify a system UI overlay style, to
   provide a hook into the style of the status bar icons on iOS.

2) Migrate the Activity service's task description API to the
   system chrome platform service. The old API will be removed
   once the Flutter repo is updated to use the new API after an
   engine roll.

flutter/flutter#3544

This patch makes the FlutterViewController interface more idiomatic by not
retaining listeners. It's the callers responsibility to make sure the lifetimes
work out.

* Close the app after the user acknowledges the Launch Error alert.

* Update to mojo 9a791294e429f9f2ca4f427796a284edb6f6768e

* Update to account for Mojo API changes

Fixes https://github.com/flutter/flutter/issues/3256

After this patch, main_ios.mm now consumes Flutter.framework via public APIs.
This patch prepares us to move Flutter on iOS over to consuming
Flutter.framework as a binary artifact.

All the headers for Flutter.framework are now in framework/Headers and all the
source files for Flutter.framework are now in framework/Source. Previously it
was unclear that FlutterAppDelegate and main_ios.mm weren't part of
Flutter.framework. (They still build as part of the framework, but that's
something I'll fix in a later patch.)

Report Flutter widgets to iOS accessibility API

These services are relative to the current view, which means they should come
in through the view services.

This patch cleans up the way we expose Mojo services into Dart. It also adds a
service provider for "view services," which will evetually contain the raw
keyboard service.

Also cleans up the messages when errors occur in both modes. I have also made the API a lot simpler and moved more of the extraneous stuff out of public. Only FlutterViewController and the Dart project configuration are publically available.

It is called each time the embedder initializes a Flutter view controller. The Mac shell (which does not have a view controller), calls it before NSApplicationMain.

Common items like the base::AtExitManager and the platform message loop are stored in `EmbedderState`

If a custom ICU data file path is specified, attempt loading from that instead of loading from the default path.

If we control the embedder, we call it just once and have it wrap UIApplicationMain.

* Remove engine code from SkySurface into the controller.
* The public API exposed by Flutter.framework for third-party embedders is present ios/public.
* Public view controller API uses idiomatic Objective-C and is subclassable by consumers.
* Paves way for moving the embedder into a framework.

Fixes #2282

Allow specification of the FLX bundle (containing no Dart snapshot) outside the iOS/Mac application bundle.

Allows launching of apps without having any Xcodebuild step in the simulator.

In the future, we'll probably want to plumb through the keyboard animation
parameters, but for now we match the behavior on Android where we move the
widgets out of the way of the keyboard without animation.

iOS: Respect view controller visibility notifications and wire in Engine::OnAppLifecycleStateChanged

We now use std::unique_ptr more consistently.

Also asserts on there being an FLX bundle at all