Teach `paints` matches about `drawRect` and how to `paintChild`.

Also, improve test coverage.

Fixes #7375

On Android and Fuchsia, default to Roboto. On iOS, use San Francisco.

Fixes #5916

This API is the front-end part of the work on showing licenses.

Future patches will:

* Provide an API for registering what licenses should be shown here,
  which will be used by this feature to shown licenses but could also be
  used by custom code for showing licenses (e.g. for people not using
  the Material widgets).

* Actually populate this license API from all the licenses we currently
  use in the engine, in the framework, and from any pub packages that
  are used (directly or indirectly) by the application.

It's TODONE

1. Use the primary color brightness to determine the color of the status bar.
    The status bar overlaps the toolbar, which has the primary color.
 2. Use Roboto for text.

This patch addresses late-breaking comments on previous patches.

They're actually leading values rather than line height values, which means
they're operative between paragraphs not within a given paragraph.

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.

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.