[monorepo] Use new location of Dart SDK source checkout in roller

Bug: https://github.com/flutter/flutter/issues/143335
Change-Id: I9614e351dc289c741446b737d5cffbb299cf8dc6
Reviewed-on: https://dart-review.googlesource.com/c/recipes/+/361920
Reviewed-by: Alexander Thomas <athom@google.com>
Commit-Queue: William Hesse <whesse@google.com>
4 files changed