(These are changes cherry-picked from in-flight branches since they are
more independent and could be helpful even without those changes.)
- Change RouteBuilder's signature to take a single argument in which the
other fields are placed, so that we can keep iterating on those
arguments without having to break compatibility each time. Also, this
makes defining route builders much simpler (only one argument to
ignore rather than a variable number).
- Expose the next performance to RouteBuilders, since sometimes the
route itself might not be where it's used.
- Allow BuildContext to be used to walk children, just like it can for
ancestors
- Allow BuildContext to be used to get the Widget of the current
BuildContext
- Allow StatefulComponentElement to be referenced with a type
specialisation so that you don't have to cast when you know what the
type you're dealing with actually is.
