Build using pub transformers

[austincummings]

It would be great if rather than running dart tool/build.dart we could just run pub build and have the work that is currently done by package:atom/build.dart be done in a transformer. This should allow for use with other packages that use transformers like polymer or angular.

[azenla]

I will consider that :)

[austincummings]

I'm messing around in a fork to allow for building with polymer. Polymer is making things very complicated though.

[azenla]

@austincummings Hmm, that should be interesting.