Updates the corelib to the new default specification for interfaces and new factory syntax

corelib/src/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.
 
 /**
  * The StringBuffer class is useful for concatenating strings
  * efficiently. Only on a call to [toString] are the strings
  * concatenated to a single String.
  */
-interface StringBuffer factory StringBufferImpl {
+interface StringBuffer default StringBufferImpl {
   /**
    * Creates the string buffer with an initial content.
    */
   StringBuffer([Object content]);
 
   /**
    * Returns the length of the buffer.
    */
   int get length();
 
@@ skipping 15 matching lines @@
   /**
    * Clears the string buffer. Returns [:this:].
    */
   StringBuffer clear();
 
   /**
    * Returns the contents of buffer as a concatenated string.
    */
   String toString();
 }