* Updated DartCallbackCache to write callback cache to disk which is
restored on engine startup
* Ensure cache isn't moved off disk in iOS

* Move the handling of delegating AppDelegate callback out of FlutterAppDelegate.

Also moves the plugin registry to FlutterViewController. So each view-controller will handle its
own plugins.

This is intended to simplify including one or more Flutter views in an existing iOS app and giving
more precise control of plugin registration.

Fixes: https://github.com/flutter/flutter/issues/16539

* formatting

* Update license golden file

* Fixed type error

* FREEZE.unindexed

* Fix Header types

* Revert "FREEZE.unindexed"

This reverts commit bebb70056c9bcb90b4321bdc2873896623ed6faa.

* Correctly added application callback for dynamic links

* Misspelling

* Revert "Added application callback (#5369)"

This reverts commit a1a54ad1.

* Added application callback

* Formatting

This reverts commit 597fa9ed as it broke
mac engine build bot.

* Move the handling of delegating AppDelegate callback out of FlutterAppDelegate.

Also moves the plugin registry to FlutterViewController. So each view-controller will handle its
own plugins.

This is intended to simplify including one or more Flutter views in an existing iOS app and giving
more precise control of plugin registration.

Fixes: https://github.com/flutter/flutter/issues/16539

* formatting

* Update license golden file

* Added handleEventsForBackgroundURLSession and
performFetchWithCompletionHandler handlers in FlutterAppDelegate to
allow for plugins to perform background downloads and fetch small
amounts of data opportunistically.

Place all iOS code that relies on APIs introduced in iOS versions later
than our base SDK version (iOS 8) behind @Available checks. This allows
us to enable the -Wunguarded-availability compiler flag for iOS builds
in the buildroot repo.

* format

* license script adaptions

* updated licenses

* review comments

* Fix remaining ftl->fxl conversions

The previous scripting pass at this did not account for objective c file endings

* Update tonic DEPS reference to the post-fxl version

Let the FlutterAppDelegate's messenger be customizable if the rootViewController isn't a FlutterViewController (#3916)

* Customizable messenger

* rename

Both the following, which we also support, are deprecated in UIKit:

* `application:handleOpenURL:`
* `application:openURL:sourceApplication:annotation:`

Expose performActionForShortcutItem call to all registered plugins so we can handle quick actions (#3763)

* Expose performActionForShortcutItem call to all registered plugins so we can handle quick actions

* Should return BOOL not void

* clang-formatted

I weaksauced

* Add debug mode background

* Review notes

The Google style guide specifies a limit of 100 columns.
https://google.github.io/styleguide/objcguide.xml?showone=Line_Length#Line_Length

The Chromium style guide specifies 80.
https://chromium.googlesource.com/chromium/src/+/master/styleguide/objective-c/objective-c.md

Provide a default status bar tap handler for FlutterAppDelegate.
By default, taps are passed to the key window's rootViewController if
it's a FlutterViewController. Apps with custom app delegates can
customize this behaviour to send the tap to the FlutterViewController(s)
they prefer by overriding touchesBegan:withEvent.

* 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.

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.)

SkyShell now supports --packages instead of --package-root.

Also fixes an issue where a error would get logged even though loading from the precompiled application bundle was successful.

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.

* 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.