Change VM's string-buffer patch to use a Uin16Array as backing buffer.

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;
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
@@ skipping 13 matching lines @@
   bool get isEmpty => length == 0;
 
   /**
    * Converts [obj] to a string and adds it to the buffer.
    *
    * *Deprecated*. Use [write] instead.
    */
   @deprecated
   void add(Object obj) => write(obj);
 
+  /// Adds the contents of [obj], converted to a string, to the buffer.
   external void write(Object obj);
 
+  /// Adds the string representation of [charCode] to the buffer.
+  external void writeCharCode(int charCode);
+
   void writeAll(Iterable objects) {
     for (Object obj in objects) write(obj);
   }
 
   void writeln(Object obj) {
     write(obj);
     write("\n");
   }
 
   /**
    * Adds the string representation of [charCode] to the buffer.
    *
    * *Deprecated* Use [writeCharCode] instead.
    */
   @deprecated
   void addCharCode(int charCode) {
     writeCharCode(charCode);
   }
 
-  /// Adds the string representation of [charCode] to the buffer.
-  void writeCharCode(int charCode) {
-    write(new String.fromCharCode(charCode));
-  }
-
   /**
    * Adds all items in [objects] to the buffer.
    *
    * *Deprecated*. Use [writeAll] instead.
    */
   @deprecated
   void addAll(Iterable objects) {
     for (Object obj in objects) write(obj);
   }
 
   /**
    * Clears the string buffer.
    *
    * *Deprecated*.
    */
   @deprecated
   external void clear();
 
   /// Returns the contents of buffer as a concatenated string.
   external String toString();
 }