merge class extensions from patch files

test/generated_sdk/lib/core/string_buffer.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 dart.core;
 
 /**
  * A class for concatenating strings efficiently.
  *
  * Allows for the incremental building of a string using write*() methods.
@@ skipping 53 matching lines @@
 
   /**
    * Clears the string buffer.
    */
   void clear() {
     _contents = "";
   }
 
   /// Returns the contents of buffer as a concatenated string.
   String toString() => Primitives.flattenString(_contents);
+
+  String _contents;
+
+  void _writeString(str) {
+    _contents = Primitives.stringConcatUnchecked(_contents, str);
+  }
 }