Changed iOS channels to start cleaning up the accessibility handler when the bridge is deleted (#19556)

Started cleaning up the accessibility handler when the bridge is
deleted and made nilling out channels safer by making sure they don't
overwrite newly setup handlers.

Added "unrecognized-selector" errors to match g3 builds, added
"overriding-method-mismatch" to catch a common error.

Test failure:

    [ RUN      ] FlutterStringCodec.CanEncodeAndDecodeNil
    2020-04-07 10:21:25.091 flutter_channels_unittests[26965:163315] *** Assertion failure in -[FlutterStringCodec encode:], ../../flutter/shell/platform/darwin/common/framework/Source/FlutterCodecs.mm:36
    2020-04-07 10:21:25.118 flutter_channels_unittests[26965:163315] *** Terminating app due to uncaught exception 'NSInternalInconsistencyException', reason: ''
    *** First throw call stack:
    (
    	0   CoreFoundation                      0x00007fff2ccabcf9 __exceptionPreprocess + 256
    	1   libobjc.A.dylib                     0x00007fff5785ea17 objc_exception_throw + 48
    	2   CoreFoundation                      0x00007fff2ccc6a16 +[NSException raise:format:arguments:] + 98
    	3   Foundation                          0x00007fff2ef58e11 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:] + 194
    	4   flutter_channels_unittests          0x0000000105f1cce2 -[FlutterStringCodec encode:] + 290
    	5   flutter_channels_unittests          0x0000000105f01bbb _ZN45FlutterStringCodec_CanEncodeAndDecodeNil_Test8TestBodyEv + 107
    	6   flutter_channels_unittests          0x00000001081d1732 _ZN7testing8internal38HandleSehExceptionsInMethodIfSupportedINS_4TestEvEET0_PT_MS4_FS3_vEPKc + 146
    	7   flutter_channels_unittests          0x00000001081a76b0 _ZN7testing8internal35HandleExceptionsInMethodIfSupportedINS_4TestEvEET0_PT_MS4_FS3_vEPKc + 128
    	8   flutter_channels_unittests          0x00000001081a75c1 _ZN7testing4Test3RunEv + 209
    	9   flutter_channels_unittests          0x00000001081a8415 _ZN7testing8TestInfo3RunEv + 229
    	10  flutter_channels_unittests          0x00000001081a95aa _ZN7testing9TestSuite3RunEv + 266
    	11  flutter_channels_unittests          0x00000001081b4e43 _ZN7testing8internal12UnitTestImpl11RunAllTestsEv + 995
    	12  flutter_channels_unittests          0x00000001081d8572 _ZN7testing8internal38HandleSehExceptionsInMethodIfSupportedINS0_12UnitTestImplEbEET0_PT_MS4_FS3_vEPKc + 146
    	13  flutter_channels_unittests          0x00000001081b49b3 _ZN7testing8internal35HandleExceptionsInMethodIfSupportedINS0_12UnitTestImplEbEET0_PT_MS4_FS3_vEPKc + 131
    	14  flutter_channels_unittests          0x00000001081b4835 _ZN7testing8UnitTest3RunEv + 197
    	15  flutter_channels_unittests          0x0000000105f22e73 _Z13RUN_ALL_TESTSv + 35
    	16  flutter_channels_unittests          0x0000000105f22d79 main + 553
    	17  libdyld.dylib                       0x00007fff5908c3d5 start + 1
    )
    libc++abi.dylib: terminating with uncaught exception of type NSException
    [ERROR:flutter/fml/backtrace.cc(110)] Caught signal SIGABRT during program execution.
    Frame 0: 0x7fff591316a6 abort
    Frame 1: 0x7fff560ea641 __cxa_bad_cast
    Frame 2: 0x7fff560ea7df default_unexpected_handler()�
    Frame 3: 0x7fff57860ee3 _objc_terminate()�
    Frame 4: 0x7fff560f619e std::__terminate(void (*)())�
    Frame 5: 0x7fff560f5f86 __cxa_get_exception_ptr
    Frame 6: 0x7fff560e8f99 __cxa_get_globals
    Frame 7: 0x7fff5785eb51 objc_exception_throw
    Frame 8: 0x7fff2ccc6a16 +[NSException raise:format:arguments:]
    Frame 9: 0x7fff2ef58e11 -[NSAssertionHandler handleFailureInMethod:object:file:lineNumber:description:]
    Frame 10: 0x105f1cce2 -[FlutterStringCodec encode:]
    Frame 11: 0x105f01bbb FlutterStringCodec_CanEncodeAndDecodeNil_Test::TestBody()�
    Frame 12: 0x1081d1732 void testing::internal::HandleSehExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*)�
    Frame 13: 0x1081a76b0 void testing::internal::HandleExceptionsInMethodIfSupported<testing::Test, void>(testing::Test*, void (testing::Test::*)(), char const*)�
    Frame 14: 0x1081a75c1 testing::Test::Run()�
    Frame 15: 0x1081a8415 testing::TestInfo::Run()�
    Frame 16: 0x1081a95aa testing::TestSuite::Run()�
    Frame 17: 0x1081b4e43 testing::internal::UnitTestImpl::RunAllTests()�
    Frame 18: 0x1081d8572 bool testing::internal::HandleSehExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*)�
    Frame 19: 0x1081b49b3 bool testing::internal::HandleExceptionsInMethodIfSupported<testing::internal::UnitTestImpl, bool>(testing::internal::UnitTestImpl*, bool (testing::internal::UnitTestImpl::*)(), char const*)�
    Frame 20: 0x1081b4835 testing::UnitTest::Run()�
    Frame 21: 0x105f22e73 RUN_ALL_TESTS()�
    Frame 22: 0x105f22d79 main
    Frame 23: 0x7fff5908c3d5 start

This reverts commit d1c90b42.

"unrecognized-selector" and "overriding-method-mismatch" added as errors.

1. Simple "GPU thread" to "raster thread" replacement.

2. Regex replace "GPU([\n\r\s]+//+ thread)" with "raster$1".

3. Regex replace "// gpu$" with "// raster".

4. Simple test change.

5. Run ci/format.sh

- added tools to help us document Flutter.framework for iOS
- filled in FlutterCodecs.h's missing documentation

* Expanded channel buffer resize to method channels.

Made it so you can resize channel buffers by sending messages to them.

Cleanup: Made a macro to assert ARC is enabled (and its inverse).

Added unit tests for method channels.

* [macos] Reland function refactor

* Fix bug

This reverts commit bd387021.

* [macos] Revert check on FlutterCodecs and refactor message function]

* Remove duplicated code

* Move FlutterPlatformMessage initialization

* Add reply to binary messenger

* Address comments

* Formatting

* Address comments

Fixed memory leak by way of accidental retain on implicit self.

This reverts commit 2f36285b.

This was crashing the integration test `- (void)testDualFlutterView`.

Created FlutterSurrogateBinaryMessenger to make sure that channels are
holding onto engines and not viewcontrollers.  This doesn't change the
public API but makes clients do what we want them to be doing, using
Engine for FlutterBinaryMessenger.

When the macOS framework was initially landed, it included some iOS code
using a localized hack. This reorganizes the code structure to clearly
indicate which files are shared between iOS and macOS, vs. those that
are platform-specific.