Make Material actually create material, with opinions about what that
means.

Make FloatingActionButton use this.

Make Scrollable use this.

Make BoxDecoration support drawing a circle instead of a rectangle, so
that floating action button doesn't need a custom painter.

Implement RaisedButton (and remove button.dart, since there's no
"button" in Material Design).

Make InkWell have a "child" argument instead of "children", and not
have it introduce a Flex into the hierarchy.

Update container.dart example. Clean up some imports.

R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1179713004.