Previously, ExpansionPanel would do weird things if interacted with
when it was already animating. This is fixed and there's now a test.

Also:

 * Minor fixes to make the gallery work in RTL, not that there's
   any way to see that without hard-coding the framework to RTL.
   But at least I'll be less annoyed when doing that.

 * Some trivial code and documentation cleanup.

* _SaltedKey solution to `ExpansionPanelList`

_SaltedKey implementation courtesy of @Hixie
Tested and confirmed working.
Fixes #11166

* Added a simple test

* Style correction to test

Fix a bunch of obvious RTL bugs from code inspection.

Fixes #11855

* more widget const constructors

* prefer const constructors

* address review comments

Nobody knew what a Block was.

Covers lib/ in package:flutter.

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

Added a widget that gives fine control over the use of steppers.
At any single moment, one step is shown with it own controls and
content.

Renamed expansion_panels.dart to expansion_panel.dart.

Added a widget that cross fades two children while animating the
size of the parent based on the children's interpolated sizes.

Added ExpansionPanel and ExpansionPanelList. The implementation
is based on the Material design spec, without enforcing every
facet of it, leaving the developer using this the liberty to
enforced it lower down the widget tree. This decision was made
based on the argument that implementing more of the spec would
reduce flexibility to a point where some design would not be
implementable with this widget anymore.