Move fitness dialog into its own class

Use the navigator to stack dialogs.

Fix test breakage

This removes the need to manually include the dialog builder in the main window's build() function.
It also removes the need to track if a dialog is visible.

Other changes:
- I made dialog.dart a bit more readable.
- I renamed transitionFinished to fullyOpaque since that's what actually matters.
- I made Routes track if they're opaque. Eventually this should probably be more configurable when creating the route.

Directions for Future Research:
- Use this for focus management somehow.
- The popup menu should use something like this.
- We should factor the following out into a showDialog() function that returns a future for the dialog's exit result:
    navigator.push(new DialogRoute(builder: (navigator, route) { ... }));
- Maybe navigator.pop() should take a value to return to that Future.

Examples as libraries

Make the Mac target deps look similar to iOS

Fixes Mac builds

Optimizations for Particle Systems

Create //sky/packages/workbench

Converted demo launcher example to library

Converted fitness example to library

Converted hello world example to library

Converted mine digger example to library

Converted stocks example to library

Conflicts:
	sky/sdk/example/game/lib/particle_system.dart

Use Skia's drawAtlas now that we've rolled to a newer Skia

Dismissable animation simplifications; added backgrounds to CardCollection cards

Roll was ff8bbe4e
This is another step in fixing #138

R=viktork@google.com

Roll Skia to the latest revision to pick up drawAtlas fix

R=chinmaygarde@google.com

New item dialog for fitness app and other minor improvements

R=eseidel

The workbench package exists to pull in pub packages from both pub.dartlang.org
and from the source tree (using dependency overrides). The idea is that
workbench will reflect a typical Sky developer's environment so that we can use
the same tools as a typical sky developer by running them with the workbench as
the cwd.

Teach skyx where the material design icons are

Add back some DEPS

clarify what is currently supported

Turns out if we don't have these deps, nothing builds these packages.

TBR=abarth@google.com

This script is unused.

Remove the magic from the Sky package

Make it possible to specify which sky app and sky server to use from the command line, overridding the Info.plist.

We now just symlink the lib directory into the packages directory in the output
directory.

Sky app in the simulator.