[macOS] Remove view ID from public (#41712)

This is a reland of #39958, but
only contains the minimal changes that removes all references to viewId
from public interfaces, reverting these changes from
#39576.

Flutter doesn't really support multi-view anyway. These methods
currently only works for view 0. We better remove them until we mark
multi-view stable. It was a mistake that I decided to add them to public
interfaces.

For the reason why #39958 failed,
I'm pretty sure it's the change to
[platform_dispatcher.dart](https://github.com/flutter/engine/pull/39958/files#diff-57d6953e215d0e5dd7260ee60b665c812aa730566ef0b30f7ff1e3d661143585)
that mysteriously and unintentionally appeared in the change list.

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Objective-C, Java style guides].
- [ ] I listed at least one issue that this PR fixes in the description
above.
- [ ] I added new tests to check the change I am making or feature I am
adding, or Hixie said the PR is test-exempt. See [testing the engine]
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Author: dkwingsmt

shell/platform/darwin/macos/framework/Headers/FlutterEngine.h

@@ -17,17 +17,6 @@
 
 // TODO: Merge this file with the iOS FlutterEngine.h.
 
-/**
- * The view ID for APIs that don't support multi-view.
- *
- * Some single-view APIs will eventually be replaced by their multi-view
- * variant. During the deprecation period, the single-view APIs will coexist with
- * and work with the multi-view APIs as if the other views don't exist.  For
- * backward compatibility, single-view APIs will always operate on the view with
- * this ID. Also, the first view assigned to the engine will also have this ID.
- */
-extern const uint64_t kFlutterDefaultViewId;
-
 @class FlutterViewController;
 
 /**

shell/platform/darwin/macos/framework/Headers/FlutterPluginRegistrarMacOS.h

@@ -44,11 +44,6 @@ FLUTTER_DARWIN_EXPORT
  */
 @property(nullable, readonly) NSView* view;
 
-/**
- * The `NSView` associated with the given view ID, if any.
- */
-- (nullable NSView*)viewForId:(uint64_t)viewId;
-
 /**
  * Registers |delegate| to receive handleMethodCall:result: callbacks for the given |channel|.
  */

shell/platform/darwin/macos/framework/Headers/FlutterViewController.h

@@ -53,16 +53,6 @@ FLUTTER_DARWIN_EXPORT
  */
 @property(nonatomic, nonnull, readonly) FlutterEngine* engine;
 
-/**
- * The identifier for this view controller.
- *
- * The ID is assigned by FlutterEngine when the view controller is attached.
- *
- * If the view controller is unattached (see FlutterViewController#attached),
- * reading this property throws an assertion.
- */
-@property(nonatomic, readonly) uint64_t viewId;
-
 /**
  * The style of mouse tracking to use for the view. Defaults to
  * FlutterMouseTrackingModeInKeyWindow.

shell/platform/darwin/macos/framework/Source/FlutterViewController_Internal.h

@@ -13,6 +13,16 @@
 
 @interface FlutterViewController () <FlutterKeyboardViewDelegate>
 
+/**
+ * The identifier for this view controller.
+ *
+ * The ID is assigned by FlutterEngine when the view controller is attached.
+ *
+ * If the view controller is unattached (see FlutterViewController#attached),
+ * reading this property throws an assertion.
+ */
+@property(nonatomic, readonly) uint64_t viewId;
+
 // The FlutterView for this view controller.
 @property(nonatomic, readonly, nullable) FlutterView* flutterView;