[swift2objc] Add support for mutating functions

[nikeokoronkwo]

Fixes #1737

The changes here address support for mutating functions.
Not much needs to be done in the transformation end, as mutating functions are only allowed on structs and protocols.

[github-actions]

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// Copyright (c) 2025, the Dart project authors. Please see the AUTHORS file
// for details. All rights reserved. Use of this source code is governed by a
// BSD-style license that can be found in the LICENSE file.

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pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[liamappelbe]

Needs formatting.

[liamappelbe]

Now that the check is in here, you can make it more specific. If a mutating keyword appears anywhere in the function declaration, this will mark the entire function as mutating. It might be the case that there it's not syntactically valid for mutating to appear anywhere else, but I don't know that's true, and it might not be true in future.

See the chunk of code that checks for annotations? You can copy it to the start of this function and use it to check for annotations that come before the (, replacing the final openParen = tokens.indexWhere( stuff, and store them in a prefixAnnotations set or something. Better yet, factor out that chunk of code into a separate function that prefixAnnotations and annotations both use.

Then this line becomes mutating: prefixAnnotations.contains('mutating'),.