"pub build" integration

[DanTup]

pub build is for packing up an app ready to deploy. It runs all transformers, which includes converting Dart code into JavaScript so that it can run in standard modern browsers. It delets the build folder (if exists) and then builds the packaged version there. The build folder would generally be excluded from source control and instead used as the package for deployment.

Although i's called build, I don't know if it's close enough to a normal build to be hooked into any build actions in VS; it's more like a "publish".

pub build would always be executed in the project root.