* Uses a special cursor value MouseCursor.defer to mark deferring. MouseTrackerAnnotation.cursor is now non-null. The mouseCursor argument of widgets or render objects can be null, which indicates using the default value.
* Moves SystemMouseCursors.uncontrolled to MouseCursor.uncontrolled.
* Changes how MouseCursor.debugDescription is defined. Previously MouseCursor.toString returns $runtimeType($debugDescription), while now it returns $debugDescription. Implementations of classes are updated, except for the ones of MouseCursor.defer and MouseCursor.uncontrolled are simply "defer" and "uncontrolled".