Add optional start index to Pattern.allMatches.

sdk/lib/core/regexp.dart

Index: sdk/lib/core/regexp.dart
diff --git a/sdk/lib/core/regexp.dart b/sdk/lib/core/regexp.dart
index b1e7f4b9134d6f92f880b96b5d40e43dc44caea8..049ff9744770f636a544b185aa6187c873a4bdc6 100644
--- a/sdk/lib/core/regexp.dart
+++ b/sdk/lib/core/regexp.dart
@@ -135,8 +135,10 @@ abstract class RegExp implements Pattern {
 
   /**
    * Returns an iterable of the matches of the regular expression on [input].
+   *
+   * If [start] is provided, only start looking for matches at `start`.
    */
-  Iterable<Match> allMatches(String input);
+  Iterable<Match> allMatches(String input, [int start = 0]);
 
   /**
    * Returns whether the regular expression has a match in the string [input].