Add String.replaceFirstMapped.

sdk/lib/core/string.dart

Index: sdk/lib/core/string.dart
diff --git a/sdk/lib/core/string.dart b/sdk/lib/core/string.dart
index e4a1c1d5c9712f50e45c9578eaa7e1e063ddb3d1..625446fb8df99ec859c2052dbd6f32b0138ebb1b 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -414,7 +414,7 @@ abstract class String implements Comparable<String>, Pattern {
   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'
@@ -423,6 +423,20 @@ abstract class String implements Comparable<String>, Pattern {
   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