[dart2js] Add blocked info to null safety candidates tool.
[dart2js] Add blocked info to null safety candidates tool.

Calculates and prints the number of files blocked by each unmigrated file. Adds he option for a flag to sort by number of blocking files.

Change-Id: I565f565d2d68e6d630bb2c9bb6d7fc46b016e365
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/251180
Commit-Queue: Nate Biggs <4bdbe5a28604b503f0b7346ee4d58c334975887b@google.com>
Reviewed-by: Sigmund Cherem <beaea8fd7d2a56555046ba4f362c8a93e3938576@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