Improve Swift usage in platform_channel_swift some more

Make a bunch of changes that willlarche requested in
https://github.com/flutter/flutter/pull/21712:

* Use `static let` within `enum` namespaces for String constants.
* Use `guard` for early exits and to avoid forced unwrapping.
* Explicitly qualify a selector.
* Fix a retain cycle by using a weak reference.

This reverts commit 11350fed7f9d2088232354a131ca35a6e0aa6307.

https://github.com/flutter/engine/compare/38a646e14cc2...9d4b80a60cb5

git log 38a646e14cc2..9d4b80a60cb5 --no-merges --oneline
9d4b80a60 Roll src/third_party/skia 17282da3aa94..5767fc042834 (8 commits) (flutter/engine#6348)
ef98dcb11 Add support for counter timeline traces from the engine. (flutter/engine#6315)
a961e96d1 Roll buildroot to pick up updated vs toolchain. (flutter/engine#6346)
d6bb59978 Fixed IsolateNameServer documentation (flutter/engine#6344)
0c854ac01 Roll src/third_party/skia bdf1431686c2..17282da3aa94 (4 commits) (flutter/engine#6345)
e394ea1dd Roll src/third_party/skia c05302bc84da..bdf1431686c2 (1 commits) (flutter/engine#6343)
bcdfc2312 Roll src/third_party/skia 5457141b011e..c05302bc84da (1 commits) (flutter/engine#6342)
a861f37f7 Pass on the new unsafe-package-serialization option (flutter/engine#6331)
5e77a1e46 Roll src/third_party/skia bd03b54cb98d..5457141b011e (1 commits) (flutter/engine#6340)
6332db875 Roll src/third_party/skia f18c297cfb74..bd03b54cb98d (1 commits) (flutter/engine#6338)
a08bc52e8 Roll src/third_party/skia 7ffa40cedbc4..f18c297cfb74 (6 commits) (flutter/engine#6336)
8247ce26b Implement restore functions on Android and iOS (flutter/engine#6322)
2ead5c06c Roll src/third_party/skia 4ef464cd3c2e..7ffa40cedbc4 (10 commits) (flutter/engine#6334)

* updated Flutter Doctor grammar

* updated messaging based on a suggestion by Greg Spencer

* grammar update

Microphone, clock, time, padlock and eye icons added (both outlined and solid)

In long lists this resulted in the dropdown scrolling to the very last
item in its list. Now clamping the value at `0.0`. Added a test to
verify that the selected item aligns with the button to test the offset.

Fixes flutter/flutter#15346

Make platform_channels_swift use more idiomatic Swift

* Remove semicolons.
* `if (condition)` => `if condition`.
* `Class.init(...)` => `Class(...)`.
* Remove `break` statements from `switch` cases.
* Remove some unnecessary uses of `self`.

* remove _ListenerMixin, which provides dubious value

* remove trailing whitespace

* Fix xcode_backend.sh script to support add2app

* Fix ios deployment target. Too old for new Xcode.

* Fix ios host app

* Register plugins with Flutter view

A dSYM file is created for the stripped `App.framework` and placed at `build/aot/App.dSYM`.

Reduces `App.framework` for Flutter Gallery by 6MB uncompressed, minus 23%.
Reduces `App.framework` for Hello World by 1.6MB uncompressed, minus 22%.

Fixes #4287.
Fixes #18693.
Helps with #21813.
See also #12012.

This change depends on https://dart-review.googlesource.com/c/sdk/+/76306.

* Roll Dart to 4cf2d3990bac1f1df2f524899e1560a3683f3133

Included CLs:

4cf2d3990b [VM runtime] Store Smi instance type in type test cache when using the interpreter.
a6b1a99f4c Make mixins abstract by default
ab305f2f26 Recover from duplicated declarations
4e32a356e0 Ignore synthetic elements created by the parser
fc04f56430 [infra] Remove deleted Dart 1 builders (app/precomp) from the test matrix
8477dc2131 Update status after 570fd5a7884
1652850381 Compute static types during impact computation
b4341f02db Remove unneeded method rti need
570fd5a788 Error on re-export, export and import of libraries with same name.
4b312b2b8c Support multiple superclasses in the mixin `on` clause.
8debb4e9db [fuchsia] Add zx_vmo_replace_as_executable calls
d34337aa2d Ignore noSuchMethod() for the purpose of searching concrete member implementations.
b6284b41e3 Revert "[VM] Dart_Initialize no longer crashes after Dart_Cleanup"
94481adbd4 [vm] Only include debugger/profiler slots in RawCode in product mode.
519ee905f9 [VM] Dart_Initialize no longer crashes after Dart_Cleanup
53497549ab [vm] Specialize 'new List()' on kernel AST
afd4cc9d1a [vm/tfa] Infer concrete class of int literals in TFA
a4d48f49e7 [vm/tfa] Infer concrete class of string literals in TFA
9106228a55 [vm, dwarf] Set default_is_stmt to true for compatability with dsymutil.
6ce0dc0006 dartfix stub implementation
e4eb2437b8 Look into superclassConstraints while searching for a member in interfaces.
95d37b0921 Revert "Implement inheritance/override checks from the spec."
355c73d4ee [VM] Remove support for option --no-preview-dart-2
95b8a19a20 Mixin declarations don't have supertype, fix isMoreSpecificThan().
e9586a5d14 [vm] Update workaround for Android Kernel bug (b089d4f).
ef7d144bc7 Add regression test for issue 34392.
56f6c52d58 large_class_declaration_test is slow now.
0ac3b8fff9 [fuchsia] Add missing vm:entry-point annotation
19e844ed5b [dart/fuzzing] Use current directory as SDK top (by default)
58e44c1400 Don't use ClassElementImpl for now in override checking.
62d154f6a3 [vm/bytecode] Record null-initialized fields in bytecode
70cc34df21 Custom format errors and exceptions to show traces
95831136b4 Fix NPE when accessing superclass
836a1d7a88 Implement inheritance/override checks from the spec.
d07039782a Remove redundant resolution
dba8b37d38 [vm] Benchmark for reading kernel bytecode

* Move one revision later.

Rationale:
Relatively harmless change, but at least has
builds (force builds on the older revision
will take a while again).

* Add tab bar theme.

* Add tab bar theme.

* Add tests, pass context to getters.

* update goldens from linux box

* update goldens from linux box

* Added new golden test, addressed comments

* override hashCode and == in TabBarTheme

* Fix comment typos

* Addressed Hans' comments.

* Formatting changes

* [TabBarTheme] Fixed spacing

* [TabBarTheme] Update goldens version to latest commit

It turns out that #21888 didn't really fix the link, because *this* is the README used by dartdoc for flutter.

* Prototype

* Fix paths to Flutter library resources

* Invoke pod install as necessary for materialized modules

* Add devicelab test for module use on iOS

* Remove debug output

* Rebase, reame materialize editable

* Add devicelab test editable iOS host app

* Removed add2app test section

* Refactor of cupertino/material/widgets app

* update docs 

* Update tests for const

MaterialPageRoute transitions are now defined by the Theme. Added (optional) support for Android P style page transitions.

Swap out the moon emoji used for progress spinner for a single-cell character.

The moon emoji looked cool, but couldn't be used because of bugs in xterm.js, used for VSCode's terminal, among others. The moon emoji is two character cells wide, but xterm.js doesn't advance by two cells when it adds the emoji, but does go back by two when it backspaces.

This changes us to a different character animation (dots) that is only one cell wide, and so doesn't have this problem.

This reverts commit 61cf946b.

* Make CupertinoApp and MaterialApp both use WidgetsApp for Navigator

* Make CupertinoApp and MaterialApp const constructors

* Make WidgetsApp routes aware

* Update tests

This will now make it so that the Dart VM class returns any Isolate that
matches the passed Pattern, without checking for any specific strings
like "main()"

This causes the search to skip over Isolates that would have matched.

https://github.com/flutter/engine/compare/f4aadb11cf13...74625aed323d

git log f4aadb11cf13..74625aed323d --no-merges --oneline
74625aed3 Keyboard padding detection heuristic to distringuish when to include bottom inset. (flutter/engine#6288)
617b122bd Roll Dart to a2eb050044eec93f0844667b8b6132e858467e4e. (flutter/engine#6321)


The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.

https://github.com/flutter/engine/compare/d5deda95176e...f4aadb11cf13

git log d5deda95176e..f4aadb11cf13 --no-merges --oneline
f4aadb11c Change FlutterDartProject to keep precedence when looking for the ass… (flutter/engine#6305)


The AutoRoll server is located here: https://autoroll.skia.org/r/flutter-engine-flutter-autoroll

Documentation for the AutoRoller is here:
https://skia.googlesource.com/buildbot/+/master/autoroll/README.md

If the roll is causing failures, please contact the current sheriff, who should
be CC'd on the roll, and stop the roller if necessary.