Update `String.replaceFirstMapped` documentation.

sdk/lib/core/string.dart

 // Copyright (c) 2012, 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 sequence of characters.
  *
  * A string can be either single or multiline. Single line strings are
@@ skipping 418 matching lines @@
 
   /**
    * Replace the first occurrence of [from] in this string.
    *
    * Returns a new string, which is this string
    * except that the first match of [from], starting from [startIndex],
    * is replaced by the result of calling [replace] with the match object.
    *
    * The optional [startIndex] is by default set to 0. If provided, it must be
    * an integer in the range `[0 .. len]`, where `len` is this string's length.
-   *
-   * If the value returned by calling `replace` is not a [String], it
-   * is converted to a `String` using its `toString` method, which must
-   * then return a string.
    */
   String replaceFirstMapped(Pattern from, String replace(Match match),
       [int startIndex = 0]);
 
   /**
    * Replaces all substrings that match [from] with [replace].
    *
    * Returns a new string in which the non-overlapping substrings matching
    * [from] (the ones iterated by `from.allMatches(thisString)`) are replaced
    * by the literal string [replace].
@@ skipping 342 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }