[ffigen] Separate Symbols into distinct kinds

[liamappelbe]

The main change is to give each Symbol a SymbolKind, indicating what sort of entity it's naming. Keywords are either disallowed for all SymbolKinds, or allowed only for methods and fields.

There's a related namespacing issue (mentioned here) that I thought would need to be fixed using the SymbolKind, but after some experimentation that turned out to be the wrong solution. I've left that fix in this PR since it's small. The issue was that param names that collided with method names were being renamed (eg NSData.dataWithBytes has a length param that was being renamed to length$1 to avoid colliding with NSData.length). This was technically correct¹, but annoying. The fix is to put the method name symbols in a special scope on the class, called methodNameScope.

Fixes #2054

Footnotes

1. Dart doesn't allow you to declare a variable and a function in the same scope with the same name. It's allowed in this case because the method scope is a separate scope, but since it's nested inside the class scope we're aliasing the method name. If the method implementation needed to call that other method, we'd run into issues. But this is not a problem for us because aside from a few very special cases, generated methods don't call other generated methods. So it's clearly better to allow aliasing in this case. I originally tried to use the SymbolKind to allow aliasing in a more general way, but it caused more problems than it solved. ↩

[github-actions]

PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.1.0	9.0.0-dev.0	9.0.0	✔️

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API leaks ⚠️

The following packages contain symbols visible in the public API, but not exported by the library. Export these symbols or remove them from your publicly visible API.

Package	Leaked API symbol	Leaking sources
objective_c	_FinalizablePointer	internal.dart::_ObjCReference::new::_finalizable

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License Headers ⚠️

// 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.

Files
pkgs/objective_c/lib/src/ns_input_stream.dart

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[coveralls]

coverage: 81.265%. remained the same
when pulling fa34545 on ffigen_keyword
into 7fea048 on main.