[ffigen] Migrate to extension types

[liamappelbe]

Before

// Interface
class AVAudioPlayer extends objc.NSObject {

// Protocol
interface class AVAudioPlayerDelegate extends objc.ObjCProtocolBase
    implements objc.NSObjectProtocol {

After

// Interface
extension type AVAudioPlayer._(objc.ObjCObject object$)
    implements objc.ObjCObject, objc.NSObject {

// Protocol
extension type AVAudioPlayerDelegate._(objc.ObjCProtocol object$)
    implements objc.ObjCProtocol, objc.NSObjectProtocol {

Other changes

• package:objective_c's collections can no longer directly implement the corresponding Dart collection. So I had to split out the Darty helpers into separate adapter classes.
• Renamed some APIs for clarity and consistency with jnigen and JS interop:
  • isInstance -> isA
  • castFrom -> as
  • castFromPointer -> fromPointer
  • ObjCObject -> ObjCObjectImpl
  • ObjCProtocol -> ObjCProtocolImpl
  • ObjCObjectBase -> ObjCObject
  • ObjCProtocolBase -> ObjCProtocol

Fixes #2664

[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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Changelog Entry ✔️

Package	Changed Files

Changes to files need to be accounted for in their respective changelogs.

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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: 81.764% (-0.7%) from 82.451%
when pulling 22527ef on ffigen_type_ext
into d2edd93 on main.

[HosseinYousefi]

Why are we changing these to a generic greaterThan(0)? The tests are now weaker as a result?

[liamappelbe]

Every now and then I have to switch one of these tests from a number to greaterThan(0) because of some CI flakiness (sometimes the exact number is different on different machines, or different runs on the same machine). Then with this extension type change we're holding fewer total references from Dart to ObjC, so I was going to have to change a bunch of the numbers to new values, or to greaterThan(0). Ultimately we don't really care about the specific number of references, just whether there's 0 or more than 0 references. So I figured it would be better to just switch them all to greaterThan(0), rather than going through and changing all the numbers, only to later switch some of them to greaterThan(0) when the specific number varies on CI.