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

This demo doesn't work properly in landscape and is confusing. It's
really more of a test of the material arc code.

Fixes #5710

To match the Align widget (and several other widgets).

Fixes #6101

We were trying to cram too much functionality in to the Dialog widget. Now we
have AlertDialog and SimpleDialog to cover to two different kinds of dialogs in
the spec.

This requires all AnimationController objects to be given a
TickerProvider, a class that can create the Ticker.

It also provides some nice mixins for people who want to have their
State provide a TickerProvider. And a schedulerTickerProvider for those
cases where you just want to see your battery burn.

Also, we now enforce destruction order for elements.

Added a demo of BottomNavigation to the Gallery.

Fixes #3208.

* turn verbs into nouns for gallery homepage

* shorter is better

Closes #5702

Update transitions_perf_test.dart

Update simple_smoke_test.dart

One of the drawer items was dsiabled but still clickable which
led to inconsistent icon coloring. THis commit makes this item
actionable. Fixes #5683.

Add update dialog to gallery, open source part of #4626

Some tweaks to the logo widget:

* remove bad assert
* allow text color to be configured
* make logo pretty in dark mode in gallery
* fix some docs

Instead of a PNG, the Flutter gallery widget is now drawn in code.

There's now a FlutterLogoDecoration class that paints the flutter logo
anywhere you can use a Decoration (e.g. AnimatedContainer).

There's now a FlutterLogo class that honors the IconTheme.

The About dialog box API now takes a Widget for the applicationIcon,
instead of an ImageProvider. It uses IconTheme to make the icon the
right size instead of using an Image widget.

Add padding, duration, and curve properties to the DrawerHeader.
Make the child of a DrawerHeader optional.

Clean up UserAccuntsDrawerHeader a bit.

Add some useful properties and methods to EdgeInsets.

Add some debug logic to RenderDecoratedBox to catch unpaired
save/restore calls when possible.

Make GestureDetector fill its parent if it has no children. Fixes
https://github.com/flutter/flutter/issues/5380

We don't yet know how to do this without poping during hero animations.

Now the flexible space bar computes its effect from its size rather than from
the Scaffold's animation.

These now have sorter names to make the callers less verbose.

These let you add Align widget to the inside or outside of a container.
Several customers have asked for these properties.

Fixes #4950

Some folks didn't realize these existed and asked us to add them. By
using them in examples, hopefully folks will discover them more easily.