Log test runs, detect regressions per file

[werediver]

The goal is to identify Dart source files which were parsed without a failure before, but cannot be parsed after certain changes.

To achieve that:

• Log both the successful and failed parses
• Match the set of files parsed successfully during a run against a "reference" set
  • Report regressions (was parsed, not failed)
  • Report improvements (was failed, now parsed)
  • Distinguish regressions and improvements from results for new or deleted files

[werediver]

Rejected for now.