* Make dividers one device pixel thick as defined in Material design spec

* Updated divider test to check stroke width

* Clarified dividers with 0 height in the docs

* Updated Divider.height docs according to PR feedback

* more widget const constructors

* prefer const constructors

* address review comments

Aggressively apply the const lint.

This prepares us for a CustomScrollView that takes slivers.

This patch introduces ScrollView, which is a convenience widget for using a
SliverBlock. This patch also switches a number of tests from Block to
ScrollView. Once we support more features of block (e.g., padding and
shrinkwrapping), we'll be able to move over more clients.

Removed old Stack layout and added a simple-to-extend interface for the
new drawer header. Also added a specialized UserAccountsDrawerHeader
consistent with Material Design guidelines.

Previously these widget had zero test coverage. Also, improve the
coverage tooling Mac.