Migrator: Add a summary_stats script
Migrator: Add a summary_stats script

The script's Usage text has the best description.

Change-Id: I6baf747a13a7655e40bc0158cb40ad965fa6ee3a
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/171422
Reviewed-by: Paul Berry <72d414e3a99fb120308437779d51667bcc0984eb@google.com>
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>

dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk,dart-lang/sdk