Clean up pointer handling.

Previously we tried to work around some possible badness from the engine
side, but now we are more assertive about the engine needing to do the
right thing.

packages/flutter/lib/src/rendering/binding.dart

@@ -56,10 +56,12 @@ class _PointerEventConverter {
       PointerDeviceKind kind = _pointerKindMap[datum.kind];
       switch (datum.type) {
         case PointerType.DOWN:
+          assert(!_pointers.containsKey(datum.pointer));
           _PointerState state = _pointers.putIfAbsent(
             datum.pointer,
             () => new _PointerState(position)
           );
+          assert(state.lastPosition == position);
           state.startNewPointer();
           state.setDown();
           yield new PointerAddedEvent(
@@ -96,13 +98,11 @@ class _PointerEventConverter {
           );
           break;
         case PointerType.MOVE:
-          _PointerState state = _pointers[datum.pointer];
           // If the service starts supporting hover pointers, then it must also
-          // start sending us ADDED and REMOVED data points. In the meantime, we
-          // only support "down" moves, and ignore spurious moves.
+          // start sending us ADDED and REMOVED data points.
           // See also: https://github.com/flutter/flutter/issues/720
-          if (state == null)
-            break;
+          assert(_pointers.containsKey(datum.pointer));
+          _PointerState state = _pointers[datum.pointer];
           assert(state.down);
           Offset offset = position - state.lastPosition;
           state.lastPosition = position;
@@ -129,8 +129,9 @@ class _PointerEventConverter {
           break;
         case PointerType.UP:
         case PointerType.CANCEL:
+          assert(_pointers.containsKey(datum.pointer));
           _PointerState state = _pointers[datum.pointer];
-          assert(state != null);
+          assert(state.down);
           assert(position == state.lastPosition);
           state.setUp();
           if (datum.type == PointerType.UP) {