Fixes https://github.com/flutter/flutter/issues/2369

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

It had bit-rotted. Also improve our error reporting for missing Material.

Make ancestorStateOfType() and ancestorRenderObjectOfType() support
subclassing.

Fixes https://github.com/flutter/flutter/issues/1087

After this patch, if you try to use a widget that depends on being enclosed in
a material, you now get an assert and a debugPrint if you're not inside a
material.

Fixes #243