add sourceTimeStamp to ScaleUpdateDetails (#135936)

This PR adds the ability to get the `sourceTimeStamp` from `ScaleUpdateDetails` in a `GestureScaleUpdateCallback` like so:

```dart
onScaleUpdate: (ScaleUpdateDetails details){
  print(details.sourceTimeStamp);
}
```

`sourceTimeStamp` is necessary when tracking velocity eg.

```dart
VelocityTracker tracker = VelocityTracker.withKind(PointerDeviceKind.touch);
///...
onScaleUpdate: (ScaleUpdateDetails details){
  tracker.addPosition(details.sourceTimeStamp!, details.focalPoint);
}
```

The docs say:

>Having both a pan gesture recognizer and a scale gesture recognizer is redundant; scale is a superset of pan. Just use the scale gesture recognizer.

Currently this is not entirely accurate, and should be fixed, as noted in https://github.com/flutter/flutter/issues/43833#issuecomment-548133779. This PR does not add `sourceTimeStamp` to `ScaleStartDetails` because it is more involved. Specifically, `ScaleStartDetails` can be created in `acceptGesture` which does not have access to the `PointerEvent` to get the `event.timeStamp` (https://github.com/flutter/flutter/blob/54fa25543243e3bf31af6af0c1fef6adabc1d5c1/packages/flutter/lib/src/gestures/scale.dart#L730C5-L730C5).

fixes https://github.com/flutter/flutter/issues/135873. See also https://github.com/flutter/flutter/issues/43833 which added delta and https://github.com/flutter/flutter/issues/49025 which added `numPointers` to `ScaleUpdateDetails` for the reason given above. `sourceTimeStamp` should probably be added to `ScaleStartDetails` as well because it exists in `DragStartDetails` and therefore in `onPanStart`.

I am not sure how to add tests for this, any input about this PR would be appreciated.

- [] All existing and new tests are passing.

packages/flutter/lib/src/gestures/scale.dart

@@ -100,6 +100,7 @@ class ScaleStartDetails {
     this.focalPoint = Offset.zero,
     Offset? localFocalPoint,
     this.pointerCount = 0,
+    this.sourceTimeStamp,
   }) : localFocalPoint = localFocalPoint ?? focalPoint;
 
   /// The initial focal point of the pointers in contact with the screen.
@@ -129,6 +130,12 @@ class ScaleStartDetails {
   /// recognizer.
   final int pointerCount;
 
+  /// Recorded timestamp of the source pointer event that triggered the scale
+  /// event.
+  ///
+  /// Could be null if triggered from proxied events such as accessibility.
+  final Duration? sourceTimeStamp;
+
   @override
   String toString() => 'ScaleStartDetails(focalPoint: $focalPoint, localFocalPoint: $localFocalPoint, pointersCount: $pointerCount)';
 }
@@ -148,6 +155,7 @@ class ScaleUpdateDetails {
     this.rotation = 0.0,
     this.pointerCount = 0,
     this.focalPointDelta = Offset.zero,
+    this.sourceTimeStamp,
   }) : assert(scale >= 0.0),
        assert(horizontalScale >= 0.0),
        assert(verticalScale >= 0.0),
@@ -225,6 +233,12 @@ class ScaleUpdateDetails {
   /// recognizer.
   final int pointerCount;
 
+  /// Recorded timestamp of the source pointer event that triggered the scale
+  /// event.
+  ///
+  /// Could be null if triggered from proxied events such as accessibility.
+  final Duration? sourceTimeStamp;
+
   @override
   String toString() => 'ScaleUpdateDetails('
     'focalPoint: $focalPoint,'
@@ -234,7 +248,8 @@ class ScaleUpdateDetails {
     ' verticalScale: $verticalScale,'
     ' rotation: $rotation,'
     ' pointerCount: $pointerCount,'
-    ' focalPointDelta: $focalPointDelta)';
+    ' focalPointDelta: $focalPointDelta,'
+    ' sourceTimeStamp: $sourceTimeStamp)';
 }
 
 /// Details for [GestureScaleEndCallback].
@@ -415,6 +430,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
   final Map<int, _PointerPanZoomData> _pointerPanZooms = <int, _PointerPanZoomData>{};
   double _initialPanZoomScaleFactor = 1;
   double _initialPanZoomRotationFactor = 0;
+  Duration? _initialEventTimestamp;
 
   double get _pointerScaleFactor => _initialSpan > 0.0 ? _currentSpan / _initialSpan : 1.0;
 
@@ -475,6 +491,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
   void addAllowedPointer(PointerDownEvent event) {
     super.addAllowedPointer(event);
     _velocityTrackers[event.pointer] = VelocityTracker.withKind(event.kind);
+    _initialEventTimestamp = event.timeStamp;
     if (_state == _ScaleState.ready) {
       _state = _ScaleState.possible;
       _initialSpan = 0.0;
@@ -494,6 +511,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     super.addAllowedPointerPanZoom(event);
     startTrackingPointer(event.pointer, event.transform);
     _velocityTrackers[event.pointer] = VelocityTracker.withKind(event.kind);
+    _initialEventTimestamp = event.timeStamp;
     if (_state == _ScaleState.ready) {
       _state = _ScaleState.possible;
       _initialPanZoomScaleFactor = 1.0;
@@ -690,6 +708,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
     }
 
     if (_state == _ScaleState.accepted && shouldStartIfAccepted) {
+      _initialEventTimestamp = event.timeStamp;
       _state = _ScaleState.started;
       _dispatchOnStartCallbackIfNeeded();
     }
@@ -707,6 +726,7 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
             rotation: _computeRotationFactor(),
             pointerCount: pointerCount,
             focalPointDelta: _delta,
+            sourceTimeStamp: event.timeStamp
           ));
         });
       }
@@ -721,9 +741,11 @@ class ScaleGestureRecognizer extends OneSequenceGestureRecognizer {
           focalPoint: _currentFocalPoint!,
           localFocalPoint: _localFocalPoint,
           pointerCount: pointerCount,
+          sourceTimeStamp: _initialEventTimestamp,
         ));
       });
     }
+    _initialEventTimestamp = null;
   }
 
   @override