Allow function as replacement argument to String.replaceAll.

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 ad4aefee7e1fca8b4e591dd6fed3f8de54c22e9d..d9c0587e8f47ba822b9f010940fb8d5973fbd2d6 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -99,9 +99,14 @@ interface String
 
   /**
    * Returns a new string where all occurences of [from] in this string
-   * are replaced with [to].
-   */
-  String replaceAll(Pattern from, String to);
+   * are replaced with a [String] depending on [replace].
+   *
+   * If [replace] is a [Function], it's called with the [Match] generated
+   * by the pattern, and its result is used as replacement.
+   * Otherwise, [replace] must be a [String], which is directly used as
+   * the replacement.
+   */
+  String replaceAll(Pattern from, var replace);
 
   /**
    * Splits the string around matches of [pattern]. Returns