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

Anywhere that accepted IconData now accepts either an Icon or an
ImageIcon.

Places that used to take an IconData in an `icon` argument, notably
IconButton and DrawerItem, now take a Widget in that slot. You can wrap
the value that used to be passed in in an Icon constructor to get the
same result.

Icon itself now takes the icon as a positional argument, for brevity.

ThemeData now has an iconTheme as well as a primaryIconTheme, the same
way it has had a textTheme and primaryTextTheme for a while.

IconTheme.of() always returns a value now (though that value itself may
have nulls in it). It defaults to the ThemeData.iconTheme.

IconThemeData.fallback() is a new method that returns an icon theme data
structure with all fields filled in.

IconTheme.merge() is a new constructor that takes a context and creates
a widget that mixes in the new values with the inherited values.

Most places that introduced an IconTheme widget now use IconTheme.merge.

IconThemeData.merge and IconThemeData.copyWith act in a way analogous to
the similarly-named members of TextStyle.

ImageIcon is introduced. It acts like Icon but takes an ImageProvider
instead of an IconData.

Also: Fix the analyzer to actually check the stocks app.

Use the route table to generate the list of screens to test in the smoke test.

* Gallery Smoke Test

See https://www.google.com/design/spec/style/writing.html#

* justifyContent -> mainAxisAlignment
* alignItems -> crossAxisAlignment
* FlexJustifyContent -> MainAxisAlignment
* FlexAlignItems -> CrossAxisAlignment

Fixes #231

* left -> leading (Removes an LTR bias)
* center -> title (Widget was actually centered)
* right -> actions (Removes an LTR bias, asymmetric with leading)

Fixes #2348

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

And fix the zillion issues that uncovered.

This reverts commit f41b3411, reversing
changes made to e33d8d96.

This was a bad check-in due to my mangling uploading a new version of the branch from a different machine.

This reverts https://github.com/flutter/flutter/pull/2639 and will be replaced by https://github.com/flutter/flutter/pull/2640

And fix the zillion issues that uncovered.

Rather than managing all the Material Design icons manually, we now
manage them using an icon font. The icon font contains glyphs for each
icon in an efficient vector format.

This patch updates the FLX tooling to include the MaterialIcons font and
updates the Icon widget to use the font instead of asset images.

Fixes #2313
Fixes #2218
Fixes #2009
Fixes #994

There should be no more uses of Performance or AnimatedValue in the framework
or the examples.

Providing a pre-built child is more efficient because we don't need to rebuild
the child every tick of the animation.

Also, clean up the class hierarchy for AnimationController now that
we've renamed progress to value. That means everything in the hierarchy
now has a value, include Watchable. This patch renames Watchable to
Animated<T>, which lets us use that type almost everywhere.

I've added some ducktape to modal bottom sheets to avoid having to
refactor all of Navigator to use AnimationController. I'll remove the
ducktape in the next patch.

These now use Animation and AnimationController instead of PerformanceView and
Performance.

Introduces a new Tooltip class.
Adds support for tooltips to IconButton and Scaffold.
Adds some tooltips to various demos.

Also some tweaks to stack.dart that I made before I decided not to go
down a "CustomPositioned" route.

Previously we used a positional argument for widgets that had multiple
children. Now we use a named argument that defaults to an empty list.

Fixes #241

Instead of taking a builder and a generic type, we can just take a
List<Widget>.