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

Our examples have been growing organically over time. This patch cleans
them up to illustrate specific aspects of Flutter.

Remove the SizeObserver you need to use snap alignment in lists by
having the lists provide the size themselves in the callback.

Also, remove snapAlignmentOffset since we couldn't figure out how to
explain it and it's not really needed.

This lets it cooperate with other gestures like tap.

The way I implemented this was to refactor the entire Draggable gesture
logic to use a new kind of gesture detector called
MultiDragGestureRecognizer. It works a bit like
MultiTapGestureRecognizer but for drags.

Also some tweaks to the velocity estimator.

(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.

We now have a resizer and close button. Also add some shadows and
launcher buttons.

Now that the engine lets us take the view host handle, we can actually
access the view host from Dart.

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.

Add a missing Material.

Fixes #1402

It had bit-rotted. Also improve our error reporting for missing Material.

RawKeyboardListener makes it easier to listen for raw keyboard events.

Fixes #787

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.

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.

This patch removes state from the animation system, which was causing problems
as we were scaling the use of animated values.

Now the "tween" objects are stateless and can watch animations, which creates a
new object that holds both the tween and the animation instead of mutating the
tween every tick of the animation.

This patch ports one client as a proof-of-concept.

Fixes #215

If you change the RenderObject tree between frames, you'll assert if
you subsequently hit test. So e.g. if you get two button presses back
to back, and you mutate the tree synchronously in response to the
first one, the second will assert.

This adds an onBuild callback to WidgetToRenderBoxAdapter to make it
easier to do the updates at the right time, i.e., during widget build.
It'll be called whenever you rebuild the WidgetToRenderBoxAdapter
itself, so all you have to do to use it is call setState() on whoever
is building the WidgetToRenderBoxAdapter.

This patch adds ShaderLayer and makes ShaderMask use it.

Fixes #1155

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

We use the ScrollDirection for more than just scrolling.

Fixes #151

This patch moves PageableList off HomogeneousViewport and onto RenderList and
friends, making it match the new ScrollableList.

ScrollableList2 is complete and subsumes all the use cases for the original
ScrollableList.

This patch also changed ScrollableList2 to use an Iterable instead of an
List for its children. This change lets clients map their underlying
data lazily. If the clients actually have a concrete list, we skip the
extra copy and grab the child list directly.

ScrollableList2 uses the same pattern as ScrollableGrid, which requires the
client to allocate widgets for every list item but doesn't inflate them unless
they're actually needed for the view. It improves on the original
ScrollableList by not requiring a rebuild of the whole visible portion of the
list when scrolling. In fact, small scrolls can often be handled entirely by
repainting.

We now support (vertically) scrollable grids with viewporting. If the
scroll doesn't reveal any new rows, we execute the scroll with a repaint
(i.e., no layout). If the scroll reveals a new row, we trigger a layout
to change the set of materialized children in the viewport.

This patch make grid layout much more flexible. The behavior is factored
out into a GridDelegate that's modeled after the custom layout
delegates. The patch includes a MaxTileWidthGridDelegate that implements
the old behavior and a FixedColumnCountGridDelegate that implements a
grid layout with a fixed number of columns.

Fixes #1048