This patch makes the sky package match the structure we've adopted for the
other Dart packages we produce from this repo.

We now use `pub` to manage Dart packages.

Fixes #306

TBR=johnmccutchan@google.com

We no longer use sky_server.go. This patch removes it.

We now use the Dart sky_server in shelldb.

At gclient sync time, we now run `pub get` for every Dart package except
`workbench`. This patch ensures that we have all the packages we need locally.

R=chinmaygarde@google.com

After this patch, we use pub to manage versioning and fetching the material
design icons.

- angle
- boringssl
- smhasher
- cython

Remove these dependencies required stripping down some BUILD.gn files to remove
targets we don't build.

BUG=
R=abarth@chromium.org

Review URL: https://codereview.chromium.org/1245683003 .