anchor.diff:    final options = parser.parse(args);
     final revision = options['revision'];
     final channel = options['channel'];
  -  final key = options['key'];
  -  if ([revision, channel, key].contains(null)) {
  -    print("Usage: update_homebrew.dart -r revision -c channel -k ssh_key\n"
  +  if ([revision, channel].contains(null)) {
  +    print("Usage: update_homebrew.dart -r revision -c channel [-k ssh_key]\n"
           "  ssh_key should allow pushes to ${GITHUB_REPO} on github");
       return;
     }
  -  final sshWrapper = Platform.script.resolve('ssh_with_key').toFilePath();
  -  gitEnvironment = {'GIT_SSH': sshWrapper, 'SSH_KEY_PATH': key};
  +
  +  final key = options['key'];
  +  if (key != null) {
  +    final sshWrapper = Platform.script.resolve('ssh_with_key').toFilePath();
  +    gitEnvironment = {'GIT_SSH': sshWrapper, 'SSH_KEY_PATH': key};
  +  }
   
     Chain.capture(() async {
       var tempDir = await Directory.systemTemp.createTemp('update_homebrew');
  
positive.code:   end
  
    def caveats; <<-EOS.undent
       To use with IntelliJ, set the Dartium execute home to:
          #{prefix}/Chromium.app
      EOS
    end
  
negative.code: 
      resource 'content_shell' do
        url '$urlBase/dev/release/${revisions['dev']}/$contentShellFile'
        sha256 '${hashes['dev'][contentShellFile]}'
      end
    end
  
    resource 'content_shell' do
      url '$urlBase/stable/release/${revisions['stable']}/$contentShellFile'
      sha256 '${hashes['stable'][contentShellFile]}'
    end
  
  