Now that we have all the Dart packages we need in //third_party, we can build
skyx bundles by default.

As part of this change, I've made it possible to build skyx bundles on Linux
and I've made the gn target names of the mojoms in //sky/services consistent
with each other and with //mojo/services/public.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1227973002 .