Turns out we have many immutable classes.

Fixes #6892

The old names were getting silly and started stepping on valuable namespace.

The new names are consistent and clear.

To be more consistent with other parts of the platform:

* put the binding in a binding.dart file.

* rearrange some members of the Scheduler class to be more close to
  execution order.

* factor out Priority class into its own file.

* add more dart docs.