Make failed capturing parenthesis produce null instead of "".

runtime/lib/regexp.dart

Index: runtime/lib/regexp.dart
diff --git a/runtime/lib/regexp.dart b/runtime/lib/regexp.dart
index e2f777e9ad5319301e855eb3cd85c489fba206be..6c0b741bb2340935b7818ff288311048ea117936 100644
--- a/runtime/lib/regexp.dart
+++ b/runtime/lib/regexp.dart
@@ -22,11 +22,17 @@ class JSRegExpMatch implements Match {
   }
 
   String group(int group) {
-    return str.substringUnchecked_(_start(group), _end(group));
+    if (group < 0 || group > regexp._groupCount) {
+      throw new IndexOutOfRangeException(group);
+    }
+    if ((group * _kMatchPair) < _match.length ) {
+      return str.substringUnchecked_(_start(group), _end(group));
+    }
+    return null;
   }
 
   String operator [](int group) {
-    return str.substringUnchecked_(_start(group), _end(group));
+    return this.group(group);
   }
 
   List<String> groups(List<int> groups) {