Currently debug logging of PointerEvent's properties is done with a hand-rolled toStringFull. Using Diagnosticable would make the logging more useful (since most properties are set to default values for most events, due to their not being relevant to that event), and also easier to extend in subclasses (as in PR #22762).

Because Diagnosticable's toString method has an argument, this is an API breaking change; if any subclass of PointerEvent has overridden toString, it would need to be updated. We may also want to remove toStringFull, which would be API-breaking, although we could instead forward it to toString with a very verbose log level.