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GestureRecognizer cleanup

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Description of change
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OneSequenceGestureRecognizer.addAllowedPointer() was changed to take a PointerDownEvent, like its superclass. Previously, it accepted the more general PointerEvent type, which was incorrect.

The framework only ever passes PointerDownEvent objects to addAllowedPointer(). Declaring OneSequenceGestureRecognizer.addAllowedPointer() to take the more general type was confusing, and caused OneSequenceGestureRecognizer subclasses to have to cast their argument to the right class.

The previous declaration forced OneSequenceGestureRecognizer descendants to override addAllowedPointer() like so:

dartclass CustomGestureRecognizer extends ScaleGestureRecognizer {
  @override
  void addAllowedPointer(PointerEvent event) {
    // insert custom handling of event here...
    super.addAllowedPointer(event);
  }
}

The new method declaration will cause this code to fail with the following error message:

super.addAllowedPointer(event); The argument type 'PointerEvent' can't be assigned to the parameter type 'PointerDownEvent'.
                                #argument_type_not_assignable

Code before migration:

dartclass CustomGestureRecognizer extends ScaleGestureRecognizer {
  @override
  void addAllowedPointer(PointerEvent event) {
    // insert custom handling of event here...
    super.addAllowedPointer(event);
  }
}

Code after migration:

dartclass CustomGestureRecognizer extends ScaleGestureRecognizer {
  @override
  void addAllowedPointer(PointerDownEvent event) {
    // insert custom handling of event here...
    super.addAllowedPointer(event);
  }
}