Rather than require Sky developers to have a Python environment, we should
write our tooling for Sky in Dart. This CL converts skyx.py into skyx.dart and
makes skyx.py into a wrapper for skyx.dart. We still need the Python wrapper
because gn requires that build actions are written in Python.

This code isn't wired into the main build yet. We'll need to add some more pub
packages to our buildbot environment first.

TBR=eseidel@google.com

Review URL: https://codereview.chromium.org/1213213003.