Add String.replaceRange and use it in replaceFirst{,Mapped}.

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 459 matching lines @@
    *
    *     pigLatin(String words) => words.replaceAllMapped(
    *         new RegExp(r'\b(\w*?)([aeiou]\w*)', caseSensitive: false),
    *         (Match m) => "${m[2]}${m[1]}${m[1].isEmpty ? 'way' : 'ay'}");
    *
    *     pigLatin('I have a secret now!'); // 'Iway avehay away ecretsay ownay!'
    */
   String replaceAllMapped(Pattern from, String replace(Match match));
 
   /**
+   * Replaces the substring from [start] to [end] with [replacement].
+   *
+   * Returns a new string equivalent to:
+   *
+   *     this.substring(0, start) + replacement + this.substring(end)
+   *
+   * The [start] and [end] indices must specify a valid range of this string.
+   * That is `0 <= start <= end <= this.length`.
+   * If [end] is `null`, it defaults to [length].
+   */
+  String replaceRange(int start, int end, String replacement);
+
+  /**
    * Splits the string at matches of [pattern] and returns a list of substrings.
    *
    * Finds all the matches of `pattern` in this string,
    * and returns the list of the substrings between the matches.
    *
    *     var string = "Hello world!";
    *     string.split(" ");                      // ['Hello', 'world!'];
    *
    * Empty matches at the beginning and end of the strings are ignored,
    * and so are empty matches right after another match.
@@ skipping 281 matching lines @@
         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }