Migrator: Add a progress bar; fixes #41187
Migrator: Add a progress bar; fixes #41187

The progress bar fills the width of the user's terminal, and computes
progress strictly based on files and phases, not weighing any file or
phase as greater than another.

Change-Id: Ic9840b3b66789512e0973160b3e0f069b714a4f8
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/147421
Commit-Queue: Samuel Rawlins <5cf2cac5eec2ad6d0eb2b0f4ff6af9a590ce2bf6@google.com>
Reviewed-by: Janice Collins <f6a8c838e29d3fa8813240c8bed106e760081e0e@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