This prepares us for a CustomScrollView that takes slivers.

The only remaining client of ScrollableGrid (the old version) is Pesto,
which needs AppBar integration.

Renames stretch constructor to fill for better consistency with
the rest of the framework.

Fixes #5835.

Now past halfway though material.dart by files.

This patch adds a consistent, simple description for the common pattern of
having a single child widget.

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308