Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/scanner/token.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 scanner;
 
 const int EOF_TOKEN = 0;
 
 const int KEYWORD_TOKEN = $k;
 const int IDENTIFIER_TOKEN = $a;
@@ skipping 201 matching lines @@
 
   int get hashCode => stringValue.hashCode;
 
   bool operator ==(other) {
     return other is SourceString && toString() == other.slowToString();
   }
 
   Iterator<int> get iterator => new StringCodeIterator(stringValue);
 
   void printOn(StringBuffer sb) {
-    sb.add(stringValue);
+    sb.write(stringValue);
   }
 
   String toString() => stringValue;
 
   String slowToString() => stringValue;
 
   SourceString copyWithoutQuotes(int initial, int terminal) {
     assert(0 <= initial);
     assert(0 <= terminal);
     assert(initial + terminal <= stringValue.length);
@@ skipping 288 matching lines @@
 
 const PrecedenceInfo HEXADECIMAL_INFO =
   const PrecedenceInfo(const SourceString('hexadecimal'), 0, HEXADECIMAL_TOKEN);
 
 const PrecedenceInfo COMMENT_INFO =
   const PrecedenceInfo(const SourceString('comment'), 0, COMMENT_TOKEN);
 
 // For reporting lexical errors.
 const PrecedenceInfo ERROR_INFO =
   const PrecedenceInfo(const SourceString('?'), 0, UNKNOWN_TOKEN);