Includes:
* Exclude frontend_server from the license crawl (https://github.com/flutter/engine/pull/4645)
* Make native wrapper classes non-abstract (https://github.com/flutter/engine/pull/4607)
* Restrict clang-format to C, C++, Obj-C, Obj-C++ (https://github.com/flutter/engine/pull/4654)

* Roll engine to pre-dart roll

* Roll engine to pick up updated dart

* Apply Map changes

* Move to dev.22

* Fix some analysis issues

* Silent analyzer

* More consts

* More const massaging

* Yet more const massaging

* Yet more const massaging

* Use nonconst()

This allows the scheduler library to depend on the services library
and the painting library to depend on the scheduler library without
the services library having to depend on the scheduler library.

While I was at it I also cleaned up some of the binding logic: the
licenses logic can now be overridden (and the test library does so),
and the image cache can now be overridden as well.

In order to allow chips to be properly drawn when they expand in size (without
using IntrinsicHeight), I needed a BoxDecoration shape that would be dependent
upon the rendered height of the widget. This seemed to be pretty generally
useful, so I added a new ShapeDecoration called StadiumBorder. It uses the
minimum dimension to adjust the BorderRadius of a rounded rect in the shape
decoration.

I also converted some uses of BoxShape to be case statements, updated the
chips to use the StadiumBorder decoration, and updated some of the metrics to match
the Material spec, as well as implementing lerping to and from StadiumBorder.

This avoids some code duplication that existed before and will make
further modifications easier.