dart2js: Fix small typo in warnings comments.

pkg/compiler/lib/src/warnings.dart

 // Copyright (c) 2011, 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.
 
 part of dart2js;
 
 const DONT_KNOW_HOW_TO_FIX = "Computer says no!";
 
 /**
  * The messages in this file should meet the following guide lines:
@@ skipping 22 matching lines @@
  * 6. Prefer contractions when they are in common use, for example, prefer
  * "can't" over "cannot". Using "cannot", "must not", "shall not", etc. is
  * off-putting to people new to programming.
  *
  * 7. Use common terminology, preferably from the Dart Language
  * Specification. This increases the user's chance of finding a good
  * explanation on the web.
  *
  * 8. Do not try to be cute or funny. It is extremely frustrating to work on a
  * product that crashes with a "tongue-in-cheek" message, especially if you did
- * not want to use this product to begin with with.
+ * not want to use this product to begin with.
  *
  * 9. Do not lie, that is, do not write error messages containing phrases like
  * "can't happen".  If the user ever saw this message, it would be a
  * lie. Prefer messages like: "Internal error: This function should not be
  * called when 'x' is null.".
  *
  * 10. Prefer to not use imperative tone. That is, the message should not sound
  * accusing or like it is ordering the user around. The computer should
  * describe the problem, not criticize for violating the specification.
  *
@@ skipping 2401 matching lines @@
   static String convertToString(value) {
     if (value is ErrorToken) {
       // Shouldn't happen.
       return value.assertionMessage;
     } else if (value is Token) {
       value = value.value;
     }
     return '$value';
   }
 }