Widgets that want to receive drops should include a DropTarget in their build.
Currently there's no widget for initiating a drag. Components can use the
DragController directly. In the future, we'll probably want to add a Draggable
that knows how to do some of this work automatically.

Fixes #612

Don't grey out Sky code in Atom

For some reason having |packages| in this .gitignore file is causing Atom to
grey out all the files in the Sky package. Moving the entry up to the root of
the git repo fixes the issue.

Add iOS sky_app target for the fitness app example

Reset the Dimissable fade animation on slow flings

Refactor RenderParagraph to do more work at the painting layer

And use the service in mine_digger to notify the user when they flag a bomb.

Add a compositing update phase

We need to compute whether a RenderObject has a composited descendant so that
we can decide whether to use canvas.saveLayer or to create a new composited
layer while walking down the tree during painting.

The compositing update walks the tree from the root only to places where the
tree's structure has been mutated. In the common case during an animation loop,
we won't need to visit any render object beyond the root.

This patch makes it easier to debug layerization issues.

Separating these notions makes them easier to work with because offset is
relative to the parent layer whereas size is intrinsic to the layer itself.
This patch fixes the underpainting bugs when compositing the stocks example.

This generalization will let us implement other alogorithims that need to walk
the RenderObject tree.

Add a ValueAnimation helper class for AnimationPerfomance.

Updates to game sound API

Add a compositing step to the lifecycle

Now we have the ability to draw multiple PictureLayers. We still squash all the
pictures together into a single SkPicture for simplicity. In the future, we'll
submit them to C++ separately and composite them on the GPU thread.

Adds CicularProgressIndicator and LinearProgressIndicator

Setting a ProgressIndicator's value to null (the default) makes it an "indeterminate progress" or activity indicator.

The indeterminate animations for both kinds of progress bars are essentially the same and wrong vis the Material Design spec, http://www.google.com/design/spec/components/progress-activity.html. I'll improve conformity with the visual design in a future CL.

Setting a ProgressIndicator's value to null (the default) makes it an "indeterminate progress" or activity indicator.

The indeterminate animations for both kinds of progress bars are essentially the same and wrong vis the Material Design spec, http://www.google.com/design/spec/components/progress-activity.html. I'll improve conformity with the visual design in a future CL.

Expose longpress and showpress in Listener

Removes the ability to jump to different positions in sound effects (this isn't supported on Android)

Used for AnimationPerformance with a single AnimatedValue<T> variable.

Currently we have a single PictureLayer that everyone draws into. A future
patch will teach the system to use multiple PictureLayers.

Adds rough sound support for sprites

Rename createsNewDisplayList to requiresCompositing

This patch prepares for introducing the compositing system by repurposing the
relevant parts of the createsNewDisplayList system. This patch also removes the
no-longer-relevant parts of that system.