Flutter uses its own DEPS file to checkout dart. To unblock
https://dart-review.googlesource.com/c/sdk/+/107837 Dart GN needs to not
fail to find build/config/gclient_args.gni even when flutter checks it
out. To fix this, this change just adds the same change to Flutter's
DEPS so that Flutter will generate the needed GN file for Dart. After
this change lands we can reland 107837.