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

corelib/src/string.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 String class represents character strings. Strings are
  * immutable. A string is represented by a list of 32-bit Unicode
  * scalar character codes accessible through the [charCodeAt] or the
  * [charCodes] method.
  */
-interface String extends Comparable, Hashable, Pattern factory Strings {
+interface String extends Comparable, Hashable, Pattern default Strings {
   /**
    * Allocates a new String for the specified [charCodes].
    */
   String.fromCharCodes(List<int> charCodes);
 
   /**
    * Gets the character (as [String]) at the given [index].
    */
   String operator [](int index);
 
@@ skipping 105 matching lines @@
    * where all characters  are made lower case. Returns [:this:] otherwise.
    */
   String toLowerCase();
 
   /**
    * If this string is not already all uper case, returns a new string
    * where all characters are made upper case. Returns [:this:] otherwise.
    */
   String toUpperCase();
 }