[macOS] Remove FlutterApplication class (flutter/engine#40939)

The need for the FlutterApplication NSApplication subclass was
eliminated in eef10ab310. The relevant app
termination logic was moved into FlutterAppDelegate in that patch.
Removing FlutterApplication needs to be done in three stages:
1. Migrate logic to FlutterAppDelegate (done in above patch)
2. Update Flutter tool template. Update migrator to migrate all apps
using FlutterApplication back to NSApplication. This is done in
https://github.com/flutter/flutter/pull/124173.
3. Eliminate FlutterApplication.h header since all references to it have
now been removed.

No tests since, as described above, this is the removal of unused (as of
step 2 above) code.

Issue: https://github.com/flutter/flutter/issues/30735

## Pre-launch Checklist

- [X] I read the [Contributor Guide] and followed the process outlined
there for submitting PRs.
- [X] I read the [Tree Hygiene] wiki page, which explains my
responsibilities.
- [X] I read and followed the [Flutter Style Guide] and the [C++,
Objective-C, Java style guides].
- [X] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
for instructions on writing and running engine tests.
- [X] I updated/added relevant documentation (doc comments with `///`).
- [X] I signed the [CLA].
- [X] All existing and new tests are passing.

If you need help, consider asking for advice on the #hackers-new channel
on [Discord].

<!-- Links -->
[Contributor Guide]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#overview
[Tree Hygiene]: https://github.com/flutter/flutter/wiki/Tree-hygiene
[Flutter Style Guide]:
https://github.com/flutter/flutter/wiki/Style-guide-for-Flutter-repo
[C++, Objective-C, Java style guides]:
https://github.com/flutter/engine/blob/main/CONTRIBUTING.md#style
[testing the engine]:
https://github.com/flutter/flutter/wiki/Testing-the-engine
[CLA]: https://cla.developers.google.com/
[flutter/tests]: https://github.com/flutter/tests
[breaking change policy]:
https://github.com/flutter/flutter/wiki/Tree-hygiene#handling-breaking-changes
[Discord]: https://github.com/flutter/flutter/wiki/Chat

engine/src/flutter/ci/licenses_golden/licenses_flutter

@@ -2603,7 +2603,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/ios/platform_view_ios.mm + ../../
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppDelegate.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterApplication.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterDartProject.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterMacOS.h + ../../../flutter/LICENSE
@@ -2616,7 +2615,6 @@ ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/Accessibil
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMacTest.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate_Internal.h + ../../../flutter/LICENSE
-ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterApplication.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm + ../../../flutter/LICENSE
 ORIGIN: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.h + ../../../flutter/LICENSE
@@ -5196,7 +5194,6 @@ FILE: ../../../flutter/shell/platform/darwin/ios/platform_view_ios.mm
 FILE: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.h
 FILE: ../../../flutter/shell/platform/darwin/ios/rendering_api_selection.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterAppDelegate.h
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterApplication.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterDartProject.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterEngine.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Headers/FlutterMacOS.h
@@ -5210,7 +5207,6 @@ FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/Accessibilit
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/AccessibilityBridgeMacTest.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterAppDelegate_Internal.h
-FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterApplication.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.h
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterBackingStore.mm
 FILE: ../../../flutter/shell/platform/darwin/macos/framework/Source/FlutterChannelKeyResponder.h

engine/src/flutter/shell/platform/darwin/macos/BUILD.gn

@@ -38,7 +38,6 @@ _framework_binary_subpath = "Versions/A/$_flutter_framework_name"
 # the Flutter engine source root.
 _flutter_framework_headers = [
   "framework/Headers/FlutterAppDelegate.h",
-  "framework/Headers/FlutterApplication.h",
   "framework/Headers/FlutterDartProject.h",
   "framework/Headers/FlutterEngine.h",
   "framework/Headers/FlutterMacOS.h",
@@ -58,7 +57,6 @@ source_set("flutter_framework_source") {
     "framework/Source/AccessibilityBridgeMac.h",
     "framework/Source/AccessibilityBridgeMac.mm",
     "framework/Source/FlutterAppDelegate.mm",
-    "framework/Source/FlutterApplication.mm",
     "framework/Source/FlutterBackingStore.h",
     "framework/Source/FlutterBackingStore.mm",
     "framework/Source/FlutterChannelKeyResponder.h",

engine/src/flutter/shell/platform/darwin/macos/framework/Headers/FlutterApplication.h

@@ -1,47 +0,0 @@
-// Copyright 2013 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#ifndef FLUTTER_FLUTTERAPPLICATION_H_
-#define FLUTTER_FLUTTERAPPLICATION_H_
-
-#import <Cocoa/Cocoa.h>
-
-/**
- * A Flutter-specific subclass of NSApplication that overrides |terminate| and
- * provides an additional |terminateApplication| method so that Flutter can
- * handle requests for termination in an asynchronous fashion.
- *
- * When a call to |terminate| comes in, either from the OS through a Quit menu
- * item, through the Quit item in the dock context menu, or from the application
- * itself, a request is sent to the Flutter framework. If that request is
- * granted, this subclass will (in |terminateApplication|) call
- * |NSApplication|'s version of |terminate| to proceed with terminating the
- * application normally by calling |applicationShouldTerminate|, etc.
- *
- * If the termination request is denied by the framework, then the application
- * will continue to execute normally, as if no |terminate| call were made.
- *
- * The |FlutterAppDelegate| always returns |NSTerminateNow| from
- * |applicationShouldTerminate|, since it has already decided by that point that
- * it should terminate.
- *
- * In order for this class to be used in place of |NSApplication|, the
- * "NSPrincipalClass" entry in the Info.plist for the application must be set to
- * "FlutterApplication". If it is not, then the application will not be given
- * the chance to deny a termination request, and calls to requestAppExit on the
- * engine (from the framework, typically) will simply exit the application
- * without ceremony.
- *
- * If the |NSApp| global isn't of type |FlutterApplication|, a log message will
- * be printed once in debug mode when the application is first accessed through
- * the singleton's |sharedApplication|, describing how to fix this.
- *
- * Flutter applications are *not* required to inherit from this class.
- * Developers of custom |NSApplication| subclasses should copy and paste code as
- * necessary from FlutterApplication.mm.
- */
-@interface FlutterApplication : NSApplication
-@end
-
-#endif  // FLUTTER_FLUTTERAPPLICATION_H_

engine/src/flutter/shell/platform/darwin/macos/framework/Headers/FlutterMacOS.h

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 #import "FlutterAppDelegate.h"
-#import "FlutterApplication.h"
 #import "FlutterBinaryMessenger.h"
 #import "FlutterChannels.h"
 #import "FlutterCodecs.h"

engine/src/flutter/shell/platform/darwin/macos/framework/Source/FlutterApplication.mm

@@ -1,8 +0,0 @@
-// Copyright 2013 The Flutter Authors. All rights reserved.
-// Use of this source code is governed by a BSD-style license that can be
-// found in the LICENSE file.
-
-#import "flutter/shell/platform/darwin/macos/framework/Headers/FlutterApplication.h"
-
-@implementation FlutterApplication
-@end