[fidldev] Update Dart related commands After the big move from topaz to fuchsia.git Test: python3 $FIDLMISC_DIR/fidldev/fidldev_test.py -b Test: manual testing Change-Id: I4c8b6bfde1b0314ccb08538b20fa6f5a382827f4 Reviewed-on: https://fuchsia-review.googlesource.com/c/fidl-misc/+/451916 Reviewed-by: Mitchell Kember <mkember@google.com>