[ffigen] ObjC id<Foo> -> Dart Foo

[liamappelbe]

This is the second part of #1462. API methods that accept id<Foo> are now code-genned as taking Foo instead of ObjCObjectBase. The protocol's implement method also returns that protocol instead of ObjCObjectBase.

There's a big caveat with this solution. I couldn't think of a way of capturing the semantics of id<Foo, Bar>, so I'm treating it as if it's just id<Foo>. This means that there are still cases where you can pass the wrong type of object to these methods. But I think these cases are pretty rare, and the failure case is just that the compiler is a bit too permissive, so it's probably not a big deal.

Details

• Added a new type to the ffigen type system: ObjCObjectPointerWithProtocols.
• Added logic to parse id into either ObjCObjectPointer or ObjCObjectPointerWithProtocols depending on whether it has any conforming protocols.
• I had to add the same castFrom and castFromPointer methods that interfaces have, as an escape hatch, and to support more advanced use cases that use ObjCProtocolBuilder to implement multiple methods.
• That meant that I had to make these protocols instantiatable, so now they're interface class instead of abstract interface class.
• I also added a protocol conformsTo method, analogous to interface's isInstance method. This method doesn't work for Dart constructed protocols, but I can add support for that if a user needs it.
• When parsing protocol Foo from id<Foo>, the AST has a weird shape. Instead of the AST looking like (decl -> methods/etc), it looks like (stubDecl --superProto-> decl -> methods/etc). So I had to add some logic in the protocol parser to detect this case and parse the true decl instead.

Fixes #1462

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PR Health

Breaking changes ✔️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	4.1.0	5.0.0-wip	5.0.0	✔️

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.

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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.

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

[coveralls]

coverage: 88.099% (+0.5%) from 87.598%
when pulling 567fc79 on protoid
into a36bc4a on main.