This reorders some classes so that this file makes more sense, and adds
a bunch of docs. It also makes the following changes:
* Move allElements from Instrumentation to TestWidgets. (Instrumentation
is going away.)
* Remove findElements.
* Rename byElement to byElementPredicate
* Rename byPredicate to byWidgetPredicate
* Implement _WidgetPredicateFinder so that byWidgetPredicate has good
messages
* Fix one use of byElementPredicate to use byWidgetPredicate.