anchor.diff:      if(commandInvocation == null) {
         // TODO: Move this to an earlier UsageException instead ?
         if(usage != null && usage.commands.isNotEmpty) {
  -        print('Must specify a sub-command.\n');
  -        _printHelp(usage);
  +        _handleUsageError(usage, 'Must specify a sub-command.');
         }
         return;
       }
  
positive.code:       // TODO: ArgParser.parse throws FormatException which does not indicate
        // which sub-command was trying to be executed.
        var helpUsage = e is UsageException ? e.usage : usage;
        print('$e\n');
        _printHelp(helpUsage);
        exitCode = 2;
        return;
      }
  
  
negative.code:   /// Handles successfully validated [commandInvocation].
    _handleResults(CommandInvocation commandInvocation);
  
    /// Prints help for [helpUsage].  If [error] is not null, prints the help and
    /// error to [stderr].
    // TODO: Integrate with Loggers.
    _printHelp(Usage helpUsage, [error]) {
      var isError = error == null;
      var sink = stdout;
      if(isError) {
        sink = stderr;
        sink.writeln(error);
        sink.writeln();
      }
      sink.writeln(getUsageFormatter(helpUsage).format());
    }
  
    _handleUsageError(Usage usage, error) {
      _printHelp(usage, error);
      exitCode = 2;
    }
  
    bool _checkHelp(CommandInvocation commandInvocation) {
  