Update `String.replaceFirstMapped` documentation.

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 18ee8c3cc44f524f3e306ed7b5e19dd81719af9b..bf181215406f99b76a2bd7ac83c86ef91dfa7c03 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -436,10 +436,6 @@ abstract class String implements Comparable<String>, Pattern {
    *
    * 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.
-   *
-   * 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]);