Cancel button behavior for showdatepicker/showtimepicker

Fixes #6483 

Previously we assumed the dial was square, which isn't always the case.

Fixes #6315

Fixes #988

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.

This feature is the implementation of an accessibility feature that changes the
size of text by a constant factor.

Fixes #5873

Having this base class lets classes like CustomPainter and DataTableSource be
more agnostic as to what's generating the repaints.

Fixes #5393.

Fundamentally the core problem was that we were not saying how wide a
date picker should be. It should be 330 pixels, if I'm measuring the
spec's mocks correctly.

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

Previously we supplied individual parameters to the various drag and pan
callbacks. However, that approach isn't extensible because each new
parameter is a breaking change to the API.

This patch makes a one-time breaking change to the API to provide a
"details" object that we can extend over time as we need to expose more
information. The first planned extension is adding enough information to
accurately produce an overscroll glow on Android.

We now match the spec much better, including handling dark theme.

The main thing we're still missing is the landscape layout.

Fixes #989

TextAlign applies to a whole paragraph instead of applying to an individual
text span. This patch moves the property out of TextStyle and into a separate
property on Text and RichText.

The `flutter` package gets more types!

Follow-up from: https://github.com/flutter/flutter/pull/3727.

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).

These wrappers also give us a chance to document what these services do.

Fixes #3164

Instead of using properties, TextPainter now receives min and max width as
parameters to layout. Also, this patch integrates the intrinsic sizing logic
into the main layout function, which satisfies all the existing uses cases.

* Fixed TimePicker crash and added tests

* Fixed TimePicker crash and added tests

* fixed a Lint-O

The dartdoc will continue until morale improves.

Fixes #1541

Also, fill out other documentation for these widgets.

Fixes #967

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

Fixes #231

This patch renames StatelessComponent to StatelessWidget and
StatefulComponent to StatefulWidget.

Fixes #2308

Fixes #1382

Now we just have one TextSpan class that handles both simple strings, trees of
children, and styling both. This approach simplifies the interface for most
clients.

This patch also removes StyledText, which was weakly typed and tricky to use
correctly. The replacement is RichText, which is strongly typed and uses
TextSpan.

We were using an Offset, which represented pixels/second, but it wasn't
clear to clients whether that was pixels/ms. Now we use a Velocity class
that is explict about the units.

Fixes #1510
Fixes #785

Each layer is supposed to reexport the parts of the previous layer
that are part of its API.

- In painting.dart, export from dart:ui all the Canvas-related APIs
  that make sense to be used at higher levels, e.g. PaintingStyle.

- Delete painting/shadows.dart. It was dead code.

- In rendering/object.dart, export all of painting.dart.

- In widgets/basic.dart, export all of painting.dart and
  animation.dart. Some classes in animation/ are renamed to make this
  less disruptive and confusing to the namespace.

- Split out Stocks back into an import model rather than a part model,
  so that it's easier to manage its dependencies on a per-file basis.

- Move Ticker to scheduler library.

- Remove as many redundant imports as possible now.

- Some minor nit picking cleanup in various files.

This patch removes Performance and AnimationValue now that we've ported the
framework over to AnimationController and Tween. This patch also cleans up the
names of the AnimationController classes now that they don't have to avoid
conflicts with the old animation API. Specifically, I've made the following
renames:

 * Animated -> Animation
 * Evaluatable -> Animatable
 * PerformanceStatus -> AnimationStatus

This patch is just renames and moving code around. There aren't any changes in
behavior.

After this patch, there aren't any direct uses of Performance in material.dart.

The dart bindings changed semantics in a non-compatible way.

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

After this patch, TimePicker should work correctly.

Fixes #559