[objective_c] Swift style KVO pattern

[liamappelbe]

Usage

1. Implement the Observer protocol
2. Call object.addObserver(observer, ...)
3. addObserver returns an Observation, which must be held for as long as you want to keep receiving updates
4. The Observation object holds strong refs to the Observer and the observed object, keeping them alive
5. Call observation.remove(), or allow it to go out of scope and be GC'd, and the observations will stop

Implementation notes

• Observer is an ordinary ffigen'd ObjC protocol defined in observer.h.
• addObserver is an extension method on NSObject.
• Observation is a thin wrapper around DOBJCObservation.
• DOBJCObservation is defined in observer.h/m and holds a strong reference to both the observed NSObject and the Observer. These references are dropped if .remove() is called.
• The observed NSObject internally holds a weak reference to the Observer.
• It's fine for the user's implementation of Observer to hold a strong reference to the observed NSObject. But it would create reference cycles if they held strong references, even indirectly, to the Observer or the Observation (this is pretty much the same restriction we have for any protocol implementation).

Fixes #1508

[github-actions]

PR Health

Breaking changes ⚠️

Package	Change	Current Version	New Version	Needed Version	Looking good?
objective_c	Breaking	8.0.0	8.1.0-wip	9.0.0 Got "8.1.0-wip" expected >= "9.0.0" (breaking changes)	⚠️

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Changelog Entry ✔️

Package	Changed Files

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

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 symbols
objective_c	_FinalizablePointer _Version

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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
no missing headers

All source files should start with a license header.

Unrelated files missing license headers

Files
pkgs/jni/lib/src/third_party/generated_bindings.dart
pkgs/native_doc_dartifier/example/native_doc_dartifier_example.dart
pkgs/native_doc_dartifier/lib/native_doc_dartifier.dart
pkgs/native_doc_dartifier/lib/src/native_doc_dartifier_base.dart
pkgs/objective_c/lib/src/ns_input_stream.dart

[coveralls]

coverage: 91.485% (-0.08%) from 91.56%
when pulling 798c530 on objc_kvo_swift
into b57cb5a on main.

[HosseinYousefi]

The added tests are failing on mac though

[liamappelbe]

The added tests are failing on mac though

Yeah, still trying to debug.

[liamappelbe]

Gave up trying to fix the test and just disabled the failing bits. Filed #2352