ScrollDragController should dispatch creation and disposal events. (#138800)

packages/flutter/lib/src/widgets/scroll_activity.dart

@@ -261,7 +261,17 @@ class ScrollDragController implements Drag {
        _retainMomentum = carriedVelocity != null && carriedVelocity != 0.0,
        _lastNonStationaryTimestamp = details.sourceTimeStamp,
        _kind = details.kind,
-       _offsetSinceLastStop = motionStartDistanceThreshold == null ? null : 0.0;
+       _offsetSinceLastStop = motionStartDistanceThreshold == null ? null : 0.0 {
+    // TODO(polina-c): stop duplicating code across disposables
+    // https://github.com/flutter/flutter/issues/137435
+    if (kFlutterMemoryAllocationsEnabled) {
+      MemoryAllocations.instance.dispatchObjectCreated(
+        library: 'package:flutter/widgets.dart',
+        className: '$ScrollDragController',
+        object: this,
+      );
+    }
+  }
 
   /// The object that will actuate the scroll view as the user drags.
   ScrollActivityDelegate get delegate => _delegate;
@@ -437,6 +447,11 @@ class ScrollDragController implements Drag {
   /// Called by the delegate when it is no longer sending events to this object.
   @mustCallSuper
   void dispose() {
+    // TODO(polina-c): stop duplicating code across disposables
+    // https://github.com/flutter/flutter/issues/137435
+    if (kFlutterMemoryAllocationsEnabled) {
+      MemoryAllocations.instance.dispatchObjectDisposed(object: this);
+    }
     _lastDetails = null;
     onDragCanceled?.call();
   }

packages/flutter/test/widgets/scroll_activity_test.dart

@@ -222,6 +222,19 @@ void main() {
       areCreateAndDispose,
     );
   });
+
+  test('$ScrollDragController dispatches memory events', () async {
+    await expectLater(
+      await memoryEvents(
+        () => ScrollDragController(
+          delegate: _ScrollActivityDelegate(),
+          details: DragStartDetails(),
+        ).dispose(),
+        ScrollDragController,
+      ),
+      areCreateAndDispose,
+    );
+  });
 }
 
 class PageView62209 extends StatefulWidget {