Luckily this class didn't actually need to extend its superclass, it
only implements the interface. So we can change `extends` to
`implements` and that's close enough, while allowing the class to be
mixed in again.
This patch aligns the iteration patterns used by animations and
ChangeNotifier. They now both respect re-entrant removal of listeners
and coalesce duplication registrations. (Also, ChangeNotifier
notification is no longer N^2).
Fixes#7533
* MultiTapGestureRecognizer previously would assert if there was no
competition.
* GestureArenaTeam would always select the first recongizer as the
winner even if a later recognizer actually accepted the pointer
sequence.
* debugPrintStack would fail a type check if maxFrames was non-null.
* FractionalOffset.lerp would throw a null-pointer exception if its
second argument was null.
Also, add a number of tests for previously untested lines of code.
This lets you use `Listenable.merge` without having to sanitize your
incoming list of change notifiers, in case your semantics are that
they are optional.
This introduces the key parts of a paginated data table, not including
the built-in pagination features.
* Provide more data for the data table demo, so there's data to page.
* Introduce a ChangeNotifier class which abstracts out
addListener/removeListener/notifyListeners. We might be able to use
this to simplify existing classes as well, though this patch doesn't
do that.
* Introduce DataTableSource, a delegate for getting data for data
tables. This will also be used by ScrollingDataTable in due course.
* Introduce PaginatedDataTable, a widget that wraps DataTable and only
shows N rows at a time, fed by a DataTableSource.
