Add String.replaceFirstMapped.

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 396 matching lines @@
    * index:
    *
    *     string.contains('X', 1);                  // false
    *     string.contains(new RegExp(r'[A-Z]'), 1); // false
    *
    * [startIndex] must not be negative or greater than [length].
    */
   bool contains(Pattern other, [int startIndex = 0]);
 
   /**
-   * Returns a new string in which  the first occurence of [from] in this string
+   * Returns a new string in which the first occurence of [from] in this string
    * is replaced with [to], starting from [startIndex]:
    *
    *     '0.0001'.replaceFirst(new RegExp(r'0'), ''); // '.0001'
    *     '0.0001'.replaceFirst(new RegExp(r'0'), '7', 1); // '0.7001'
    */
   String replaceFirst(Pattern from, String to, [int startIndex = 0]);
 
   /**
+   * Replace the first occurence of [from] in this string.
+   *
+   * Returns a new string, which is this string
+   * except that the first match of [pattern], starting from [startIndex],
+   * is replaced by the result of calling [replace] with the match object.
+   *
+   * 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].
    *
    *     'resume'.replaceAll(new RegExp(r'e'), 'é'); // 'résumé'
    *
    * Notice that the [replace] string is not interpreted. If the replacement
    * depends on the match (for example on a [RegExp]'s capture groups), use
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         _position = position - 1;
         _currentCodePoint = _combineSurrogatePair(prevCodeUnit, codeUnit);
         return true;
       }
     }
     _position = position;
     _currentCodePoint = codeUnit;
     return true;
   }
 }