Remove obsolete String + code

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.
  */
@@ skipping 52 matching lines @@
    * Returns whether this string is empty.
    */
   bool isEmpty();
 
   /**
    * Creates a new string by concatenating this string with [other].
    */
   String concat(String other);
 
   /**
-   * Converts [other] to a string and creates a new string by
-   * concatenating this string with the converted [other].
-   */
-  String operator +(Object other);
-
-  /**
    * Returns a substring of this string in the given range.
    * [startIndex] is inclusive and [endIndex] is exclusive.
    */
   String substring(int startIndex, [int endIndex]);
 
   /**
    * Returns a new string where leading and trailing whitespaces of
    * this string have been removed, or returns this string if it does
    * not have leading and trailing whitespaces.
    */
@@ skipping 38 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();
 }