Fix a bug with command line history.

tools/ddbg/lib/commando.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 import 'dart:math';
 
 import 'terminfo.dart';
@@ skipping 406 matching lines @@
       // Any edits get committed to history.
       _replaceHistory(_currentLine, _linePos);
     }
 
     _linePos -= 1;
     var line = _lines[_linePos];
     _update(line, line.length);
   }
 
   void _historyNext() {
-    if (_linePos == (_lines.length - 1)) {
+    // For the very first command, _linePos (0) will exceed
+    // (_lines.length - 1) (-1) so we use a ">=" here instead of an "==".
+    if (_linePos >= (_lines.length - 1)) {
       return;
     }
 
     // Any edits get committed to history.
     _replaceHistory(_currentLine, _linePos);
 
     _linePos += 1;
     var line = _lines[_linePos];
     _update(line, line.length);
   }
@@ skipping 263 matching lines @@
     print('Received command($command)');
   }
 }
 
 
 void main() {
   stdout.writeln('[Commando demo]');
   cmd = new Commando(stdin, stdout, _handleCommand,
                      completer:_myCompleter);
 }