* add trailing commas on list/map/parameters

* add trailing commas on Invocation with nb of arg>1

* add commas for widget containing widgets

* add trailing commas if instantiation contains trailing comma

* revert bad change

* enable lint unnecessary_new

* fix tests

* fix tests

* fix tests

* Bump Dart SDK to 1.25.0-dev.9.0

* add link to sdk bug

Fixes https://github.com/flutter/flutter/issues/9342

* more widget const constructors

* prefer const constructors

* address review comments

* make @immutable const

* fix build

This patch replaces uses of Flexible with Expanded where we're using
FlexFit.tight. We still need to think of a better name for the
FlexFit.loose variant.

Also, improve the docs for Row, Column, Flex, and RenderFlex to be more
problem-oriented and to give a complete account of the layout algorithn.

Fixes #6960
Fixes #5169

Since we can change the theme's platform, we need to make sure we
propagate that throughout rather than having half the app use the native
platform and half the app use the selected platform.

This patch is a warmup to improving the visuals in the calculator demo.

Related to #4535

Fixes 3544

* New Material Design calculator example. (Attempt 2 after git fetch upstream).

* Responded to code review.

* Second round of code review.