This concept is now private to AnimationController. All the clients actually
want the AnimationStatus.

We don't need these keys. Also, improve style in several places.

Fixes #2225

Also:
 - add operator==/hashCode/toString to ViewportDimensions
 - add toString to BindingBase
 - add toString and debugFillDescription to ScrollBehavior
 - fix a bug in the RawGestureDetectorState's replaceGestureRecognizers
 - rename MixedViewport's onExtentsUpdate to onExtentChanged
 - replace ExtentsUpdateCallback with ValueChanged<double>
 - remove a microtask for dispatching scroll start, since it
   did not appear to have any purpose
 - added dartdocs to Instrumentation until I understood it
 - made all event dispatch in Instrumentation drain microtasks

There's no reason to make clients supply a PointerRounter and a
GestureArena when constructing gesture recognizers. These objects are
statics and the gesture recognizers can just grab them directly.

Also, remove the callback constructor arguments. Almost no code used
them. Instead, people seem to prefer using the `..` operator to set
callbacks on the recognizers. Removing the arguments removes a bunch of
boilerplate.

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.

In the dark theme, there isn't really a primary swatch, so this API was
a sandtrap. Instead, be explicit about the colors we need for various
widgets in the theme.

Fixes #1277

These files are already in the "material" folder. They don't need to be
prefixed with material again.

Fixes #702

The example in the spec with multiple actions is an anti-example.

Fixes #1876

This patch improves the checkbox animation as suggested by the design
team. The color transition now occurs earlier in the animation and there
are fewer components to the animation.

Fixes #1614

Fixes https://github.com/flutter/flutter/issues/1331

Hopefully this will be enough to start with. If this continues to be a
point of confusion we can see about adding more documentation or
changing the API somehow.

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

This makes it match the material spec more.
https://www.google.com/design/spec/style/icons.html

Fixes https://github.com/flutter/flutter/issues/1357

Drawer doesn't need a SizeObserver, since it only looks at the size
in event handlers. It can just go and probe the tree to read the size.

Also, change from using _kEdgeDragWidth to using _kWidth when figuring
out how much of the drawer to show when dragging it from the edge, since
that is more likely to match the drawer's width.

Move StepTween to tween.dart.

Move _buildIndicator() methods to be next to the build() methods they're
used from, so that reading the code is easier.

Fix typos, add some comments, style nits.

This patch tightens up the imports in this file and hides some more of
the implementation details by making RepeatingCurveTween and StepTween
private.

Also, removed a CurvedAnimation with a linear curve.  That's not needed
because the animation controller itself is an Animation<double>.

This animation expands/collapses an arc that's 3/4ths of a circle.
The next iteration begins where the previous appeared to finish.
This happens while the whole thing rotates, such that the starting point of each
expansion moves in a star-wise pattern around the circle.

See:
https://www.google.com/design/spec/components/progress-activity.html
(note: the video here demonstrating this animation is actually slightly incorrect...
we'll hopefully update it soon)

(see https://github.com/flutter/flutter/issues/1586)

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.

These classes now share more code and have feature parity.

The virtual viewport machinery now handles all of these use cases.
Previous clients of ScrollableWidgetList can use ScrollableLazyList
instead.

Add support for the appbar behavior described in the "Flexible space with image" section of https://www.google.com/design/spec/patterns/scrolling-techniques.html#scrolling-techniques-scrolling.

Now the keyboard is responsible for maintaining the state of the text
field.