Fix warnings spotted by dart.

dart/sdk/lib/_internal/compiler/implementation/tree/dartstring.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.
 
 part of tree;
 
 /**
  * The [DartString] type represents a Dart string value as a sequence of Unicode
  * Scalar Values.
  * After parsing, any valid [LiteralString] will contain a [DartString]
@@ skipping 82 matching lines @@
   EscapedSourceDartString(source, length) : super(source, length);
   Iterator<int> get iterator {
     if (toStringCache != null) return new StringCodeIterator(toStringCache);
     return new StringEscapeIterator(source);
   }
   String slowToString() {
     if (toStringCache != null) return toStringCache;
     StringBuffer buffer = new StringBuffer();
     StringEscapeIterator it = new StringEscapeIterator(source);
     while (it.moveNext()) {
-      buffer.addCharCode(it.current);
+      buffer.writeCharCode(it.current);
     }
     toStringCache = buffer.toString();
     return toStringCache;
   }
 }
 
 /**
  * The concatenation of two [DartString]s.
  */
 class ConsDartString extends DartString {
@@ skipping 112 matching lines @@
         }
         _current = value;
         break;
       default:
         _current = code;
     }
     return true;
   }
 }