Specify --package-root for dart_analyzer (and probably other commands.)

[amouravski]

You could probably do something smart, but for now, there needs to be a setting for this otherwise it dies when it can't find certain imports.

[timothyarmstrong]

Hmm, true. We're starting to hit the limit with what we can do with the basic Sublime abilities; once we have a full plugin running though it will possible to configure stuff like this. The build system is unfortunately not configurable, unless the user makes their own every time.