Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/scanner/string_scanner.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;
 
 /**
  * Scanner that reads from a String and creates tokens that points to
  * substrings.
  */
@@ skipping 57 matching lines @@
       cashedHash = slowToString().hashCode;
     }
     return cashedHash;
   }
 
   bool operator ==(other) {
     return other is SourceString && slowToString() == other.slowToString();
   }
 
   void printOn(StringBuffer sb) {
-    sb.add(internalString.substring(begin, end));
+    sb.write(internalString.substring(begin, end));
   }
 
   String slowToString() {
     if (cachedSubString == null) {
       cachedSubString = internalString.substring(begin, end);
     }
     return cachedSubString;
   }
 
   String toString() => "SubstringWrapper(${slowToString()})";
 
   String get stringValue => null;
 
   Iterator<int> get iterator =>
       new StringCodeIterator.substring(internalString, begin, end);
 
   SourceString copyWithoutQuotes(int initial, int terminal) {
     assert(0 <= initial);
     assert(0 <= terminal);
     assert(initial + terminal <= internalString.length);
     return new SubstringWrapper(internalString,
                                 begin + initial, end - terminal);
   }
 
   bool get isEmpty => begin == end;
 
   bool isPrivate() => !isEmpty && internalString.codeUnitAt(begin) == $_;
 }