As part of this:
- A lot of classes got new lerp functions, including e.g. TextStyle.
- Theme's constructor story got overhauled. You can now configure
everything if you really want to, and we're better about defaults.
- Material no longer automatically animates its background color.
(It still does for its shadow.)
- Tabs try to get the indicator color from the theme.
- The fields in ThemeData got reordered for sanity.
- Theme.== and Theme.hashCode got fixed.
- Typography got a bit of a spring cleaning.
Fixes#613.
73c9ebab19