Make PipelineOwner a base class (#161789)

Such that nobody `implements` it. Currently it has private APIs used by
the `RenderObject` class (e.g., the dirty lists). If someone were to
implement a `PipelineOwner` outside of object.dart, they will get a
runtime `noSuchMethod` crash since they won't be able to provide those
private APIs and RenderObjects actually expect those APIs to exist.

Will be slow to update/respond due to extremely spotty internet :(

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packages/flutter/lib/src/rendering/binding.dart

@@ -827,7 +827,7 @@ class _BindingPipelineManifold extends ChangeNotifier implements PipelineManifol
 }
 
 // A [PipelineOwner] that cannot have a root node.
-class _DefaultRootPipelineOwner extends PipelineOwner {
+final class _DefaultRootPipelineOwner extends PipelineOwner {
   _DefaultRootPipelineOwner() : super(onSemanticsUpdate: _onSemanticsUpdate);
 
   @override

packages/flutter/lib/src/rendering/object.dart

@@ -961,7 +961,7 @@ class _LocalSemanticsHandle implements SemanticsHandle {
 /// without tying it to a specific binding implementation. All [PipelineOwner]s
 /// in a given tree must be attached to the same [PipelineManifold]. This
 /// happens automatically during [adoptChild].
-class PipelineOwner with DiagnosticableTreeMixin {
+base class PipelineOwner with DiagnosticableTreeMixin {
   /// Creates a pipeline owner.
   ///
   /// Typically created by the binding (e.g., [RendererBinding]), but can be

packages/flutter/test/rendering/multi_view_binding_test.dart

@@ -200,7 +200,7 @@ class FakeFlutterView extends Fake implements FlutterView {
   }
 }
 
-class PipelineOwnerSpy extends PipelineOwner {
+final class PipelineOwnerSpy extends PipelineOwner {
   @override
   final SemanticsOwnerSpy semanticsOwner = SemanticsOwnerSpy();
 }