Extend build_runner to allow it to run builders and compile with DDC.

This is the minimum amount of work required to approximately replace pub serve, and nothing more - at least for the AngularDart case. After this MVP, it should be possible to use [the latest] AngularDart, and use some sort of of serve command instead of pub serve, and get:

• Fast incremental builds with on-disk cache.
• Development and debugging using dartdevc (only).
• The ability to add your own builders/serving mechanisms (for advanced users).