* WIP

* Added tests and updated SafeArea implementation.

* Analyzer nits

* Review feedback

* Updated for SnackBar and PersistentFooterButton cases, added tests to check other potential spots.

* doc addition for SafeArea

* Typos

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

As a convenience, this adds a set of minimum padding to apply. The
greater of the minimum padding and the media padding is applied to each
edge.

A SafeArea-like widget that applies a SliverPadding instead of a
Padding.

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.

* SafeArea

* AnimatedSafeArea

* AppBar test

* Apply feedback