Update the semantics of the 'special-command' flag given to the test script.

tools/testing/dart/test_options.dart

 // Copyright (c) 2012, 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.
 
 #library("test_options_parser");
 
 #import("dart:io");
 #import("dart:math");
 #import("drt_updater.dart");
 #import("test_suite.dart");
@@ skipping 26 matching lines @@
 
 /**
  * Parser of test options.
  */
 class TestOptionsParser {
   String specialCommandHelp =
 """
 Special command support. Wraps the command line in
 a special command. The special command should contain
 an '@' character which will be replaced by the normal
-command.
+command executable.
 
-For example if the normal command that will be executed
+For example if the normal command line that will be executed
 is 'dart file.dart' and you specify special command
 'python -u valgrind.py @ suffix' the final command will be
-'python -u valgrind.py dart file.dart suffix'""";
+'python -u valgrind.py dart suffix file.dart'""";
 
   /**
    * Creates a test options parser initialized with the known options.
    */
   TestOptionsParser() {
     _options =
         [ new _TestOptionSpecification(
               'mode',
               'Mode in which to run the tests',
               ['-m', '--mode'],
@@ skipping 579 matching lines @@
         return option;
       }
     }
     print('Unknown test option $name');
     exit(1);
   }
 
 
   List<_TestOptionSpecification> _options;
 }