Refactor driver to be more testable.

[pq]

From @pq on June 16, 2015 17:8

TL;DR our test coverage is lacking; let's get to fixing that by making the Driver more test-friendly.

Copied from original issue: dart-lang/analyzer_cli#31

[pq]

Reassessing, I think we've reached a point of diminishing returns, especially given that this code will largely go away with the context manager rework (#24133).