Enforce that we get a final move to the pointer up location

Previously we asserted that we got a pointer move to the location where the
pointer up occured, but not all sources of pointer packets respect that
invariant. Specifically, on the iOS simulator, of you drag outside the window,
you'll get a stream of pointers that violates that invariant.

This patch teaches the converter to insert a PointerMoveEvent to move the
pointer to the location where the up occurs, repairing the invariant.

Fixes #1912

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

@@ -121,6 +121,36 @@ class PointerEventConverter {
           assert(_pointers.containsKey(datum.pointer));
           _PointerState state = _pointers[datum.pointer];
           assert(state.down);
+          if (position != state.lastPosition) {
+            // Not all sources of pointer packets respect the invariant that
+            // they move the pointer to the up location before sending the up
+            // event. For example, in the iOS simulator, of you drag outside the
+            // window, you'll get a stream of pointers that violates that
+            // invariant. We restore the invariant here for our clients.
+            Offset offset = position - state.lastPosition;
+            state.lastPosition = position;
+            yield new PointerMoveEvent(
+              timeStamp: timeStamp,
+              pointer: state.pointer,
+              kind: kind,
+              position: position,
+              delta: offset,
+              down: state.down,
+              obscured: datum.obscured,
+              pressure: datum.pressure,
+              pressureMin: datum.pressureMin,
+              pressureMax: datum.pressureMax,
+              distance: datum.distance,
+              distanceMax: datum.distanceMax,
+              radiusMajor: datum.radiusMajor,
+              radiusMinor: datum.radiusMajor,
+              radiusMin: datum.radiusMin,
+              radiusMax: datum.radiusMax,
+              orientation: datum.orientation,
+              tilt: datum.tilt
+            );
+            state.lastPosition = position;
+          }
           assert(position == state.lastPosition);
           state.setUp();
           if (datum.type == mojom.PointerType.up) {