Refactor JSEnv to support async running
Remove JSEnv constructor-blow up. Use default args
Add ComJSEnv and make RhinoJSEnv a ComJSEnv
Make NodeJSEnv a ComJSEnv
Clean up standalone sbt test project
Restrict files from plugin we include in partest build
Make preLinkJSEnv and postLinkJSEnv a task.

They might depend on a task such as update. On the other hand, they
are only used to run things, which arguably must be a task. Therefore,
it is safe to make them tasks.
Make PhantomJSEnv a ComJSEnv
Merge pull request #1237 from gzm0/com-js-env

Add ComJSEnv