This reverts commit 2042a0f0.

Roll back dart dependency to work around build errors

Tracking issue https://github.com/domokit/sky_engine/issues/654

Avoid disabling extra clang plugins on iOS

Trivial documentation for files in theme/.

Add a trivial README for the mojo/ directory in the SDK.

update the readme for the material design package

Remove more dead code from roll_apk_version

Also fixed the url in roll_pub_versions CHANGELOG.md message.

@abarth

Adds SoundManager class

@abarth

Split roll_versions.py into apk vs. pub

Adds a new ActionDelay class to sprites

Make it easier to run SkyShell from the command line on Mac

Now you can use --snapshot to load snapshots from the command line and have
them appear visually on Mac. Also, remove code duplication with Linux by
factoring common testing code into //sky/shell/testing.

Fix Mac build

Looks like the Keyboard service is only for iOS. We shouldn't try to build it
on Mac.

Keyboard service implementation for iOS

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