* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

This attempts to apply some of what we learnt from UX studies, namely
that people wonder how to add multiple children to widget that take
one child.

Fixes #11855

* more widget const constructors

* prefer const constructors

* address review comments

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