Look at using build_runner to generate Dart files

_From @kevmoo on February 1, 2018 15:23_

Could be useful to align with other source generation concepts in the platform

_Copied from original issue: dart-lang/grpc-dart#54_

Start by investigating how build_runner could support generating Dart files and how that would align with the platform's other source-generation concepts. Done would require an agreed scope and implementation plan, since the issue names no files, entry point, or acceptance criteria.

Written by the indexing model from the issue text.