Add file.dart to DEPS (#22794)

https://dart-review.googlesource.com/c/sdk/+/174382 added `file.dart` to the DEPS in Dart.
https://dart.googlesource.com/external/github.com/flutter/engine/+/747b7912ca04fa78c13bc1564c33d4315c4185d1
1 file changed
tree: ded9c273344c218e7ecc223ec7671a687085df97
  1. commits.json