- truncate pixel values to 1dp since there's really no point being told
  the Size is 302.98732587287 by 648.28498579187.

- describe more Widgets so that debugDumpApp() is more useful.

- remove bufferValue from ProgressIndicator (cc @hansmuller) since it's
  not yet implemented.

- half-hearted toString() for ThemeData. There's no point making a
  complete one, since it would cause line-wrap even on big monitors in
  debugDumpApp dumps, and you can easily get the actual values from a
  debugging if that's the issue.

- flesh out BoxConstraints.toString() so that fully unconstrained and
  fully infinite constraints are called out explicitly. I experimented
  with adding even more special cases, e.g. calling out unconstrained
  widths with fixed heights, etc, but it made the output less readable.

- remove a redundant _updateVariable() in AnimatedContainer (cc
  @abarth).

- add more information to RenderView.toString().

Now that you import a top-level file for each layer of the Flutter
framework, within the framework we can use relative paths without being
worried about the copy/pasta problem we used to have.

* Moves from vector_math to vector_math_64
* Adds support for Float64List in Dart bindings

Move the animation libraries into src/animation and change importers to use
package:sky/animation.dart. Also, move scheduler.dart into the animation
library so that the animation library can be self-contained.

Previously we reversed the event path by mistake before dispatching events.

Code outside of package:sky should import this code using

package:sky/rendering.dart
package:sky/widgets.dart

Moving this code into the "src" directory is a convention that signifies that
and it cleans up the generated dartdoc because the libraries in the src
directory aren't included in the generated documentation. Instead, the classes
are documented in the widgets.dart and rendering.dart libraries.

When we start, we might not have initialized the devicePixelRatio value. The
value will be initialized by the time we paint, so now we update the value
every time we paint.

We still use SkPicture as our rasterization backend, but now we're uploading
our layer tree to C++. A future patch will push the layer tree deeper into the
system and we'll eventually switch backends.

We now use the repaint system to do all the painting. During initialization, we
set up a root layer that applies the device pixel ratio.

Fixes #706

- Introduce some new Layer classes.
- Introduce paintChildWith* methods.
- Convert paint() methods to use paintChildWith* where appropriate.
- Fix paintBounds logic in Layer world.
- Introduce Layer.replaceWith(), so that it's clearer what's going on.
- Make RenderObjects have a ContainerLayer, not a PictureLayer.
- Introduce a PaintingContext.replacingLayer() constructor to highlight
  where we are creating a layer just to replace an older one.
- Rename some layer-related methods and fields for clarity:
   requiresCompositing -> hasLayer
   hasCompositedDescendant -> needsCompositing
   updateCompositing -> updateCompositingBits
   _needsCompositingUpdate -> _needsCompositingBitsUpdate
   markNeedsCompositingUpdate -> markNeedsCompositingBitsUpdate
- After updating compositing bits, if we find that the bit changed, we
  now call markNeedsPaint().
- Reorder markNeedsPaint() logic for clarity.
- Make flushPaint() start at the deepest node.
- Make _compositeChild() avoid repainting children with hasLayer that
  aren't dirty, instead it just reuses their existing layer.
- Made RenderView reuse the RenderObject layer instead of having its own.
- Made RenderView have hasLayer set to true.
- Add various asserts and comments.

Sadly, box.dart has grown much longer than 1000 lines. This patch splits it up
into several files based on the class hierarchy. Fortunately, many of these
classes are loosely coupled to each other.