Tweak comments added by `write` function on packages_file.dart.

Now adds "# " in front, not just "#", and doesn't treat the empty
string after a final "\n" as an extra line.

R=sgjesse@google.com

Review URL: https://codereview.chromium.org//1167223004.
1 file changed
tree: 2eb87a46bc5d4f695879890b9676c1dd5fbef081
  1. pkgs/