Fix a bunch of obvious RTL bugs from code inspection.

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

* more widget const constructors

* prefer const constructors

* address review comments

Rename State.config to State.widget
Rename State.didUpdateConfig to State.didUpdateWidget
Renamed all State subclasses' local variables named config to something else

* add @required when there's an assert not null

* address review comments

The latter is the proper spelling.

Fixes #8883

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

Fixes https://github.com/flutter/flutter/issues/6628

Eliminates hardcoding of Typography.white in favour of picking up the
platform default typography from ThemeData.

Previously, we were getting double padding: both inside and outside the button.
This patch moves all the padding inside the button so that the whole region is
hittable and the text is positioned correctly.

Fixes #6059

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.

In the interest of using the name in the case of Material steppers,
this commit renames Step curve to Threshold.

We now use the `@required` annotation to encourage developers to
explicitly set onPressed and onChanged callbacks to null when that would
disable the widget.

Fixes #287

Fixes 3544

For consistency.

Fixes #4142

This reverts commit 55f9145e.

Turns out that this commit breaks apps that use the material library,
because of the _errorTextStyle DefaultTextStyle which has inherit:true.
Just setting it to false doesn't work, unfortunately, because then you
hit some sort of issue with merging that text style with others that
have inherit:true.

To make it clear that this constructor requires an explicit style. Also
throw a descriptive error recommending the inherit constructor for
styles with the inherit bit set.

Fixes #3842

Making progress towards document all public APIs in material.dart. We're still
missing a few odds and ends (as well as missing docs in tabs.dart,
tooltip.dart, and two_level_list.dart).

This patch completes the circuit on this library. The next step is to go
through and add docs to anything we missed.

Instead of incorporating the margin into the button, introduce a ButtonBar
widget that supplies the proper spacing between the buttons. Also, make these
buttons more configurable via ButtonTheme so that dialogs can change the
minWidth and padding of the buttons as required by the spec.

Fixes #1843
Fixes #3184

A flow layout is optimized for reposition children with transformation
matrices. A flow layout can animate the position of its children very
efficiently.

They should automagically disable after the first tap.

Previously, the Dismissable widget was animating down to zero width off
screen. Now, we tell it to skip the resize animation.

Fixes #3030

Fixes #617

Fixes #463

Adds some names for common FractionalOffset values.

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

Fixes #231

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308