Update `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 78818b7d7555f94f5f8e135a2ce48e77e64dd453..38df590e9d7da5cd5a86c3699ba589f429a406ff 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -428,6 +428,9 @@ abstract class String implements Comparable<String>, Pattern {
    * except that the first match of [pattern], 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.

[Lasse Reichstein Nielsen, 2017/01/17 15:59:48]

Maybe just:
 in the range `0 .. len` (both inclusive)
instead of using math notation.

+   *
    * 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.