Add optional startIndex to String.replaceFirst

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 ebc6c20700299974fe4fd4336bce9b203114e2ac..384b183c0b50af170b709dedb18605cc4e5eede3 100644
--- a/sdk/lib/core/string.dart
+++ b/sdk/lib/core/string.dart
@@ -409,11 +409,12 @@ abstract class String implements Comparable<String>, Pattern {
 
   /**
    * Returns a new string in which  the first occurence of [from] in this string
-   * is replaced with [to]:
+   * 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);
+  String replaceFirst(Pattern from, String to, [int startIndex = 0]);
 
   /**
    * Replaces all substrings that match [from] with [replace].