Now with fewer lists.

Now TextStyle has a boolean "inherit" value, which controls whether null values
should use should inherit from the current default text style or whether they
should use their initial values.

The goal is to follow the guidelines in
https://github.com/flutter/engine/blob/master/sky/specs/style-guide.md#packages

Fixes #1638

Also, have dialog buttons use the accent colour.
Also, generally rationalise some of this stuff to match Material better.

Also, introduce Colors and Typography to hold the material colors and the
typography declarations. Previously we expected clients of these libraries to
import them into a namespace, but that doesn't play nice with re-exporting them
from material.dart.

These are now declared in dart:sky as part of ParagraphBuilder.