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..eea2f4bcba328396d563952c3f90e808677f9804 100644
--- a/runtime/lib/regexp.dart
+++ b/runtime/lib/regexp.dart
@@ -14,26 +14,34 @@ class JSRegExpMatch implements Match {
   }
 
   int _start(int group) {
-    return _match[(group * _kMatchPair)];
+    return _match[(group * MATCH_PAIR)];
   }
 
   int _end(int group) {
-    return _match[(group * _kMatchPair) + 1];
+    return _match[(group * MATCH_PAIR) + 1];
   }
 
   String group(int group) {
-    return str.substringUnchecked_(_start(group), _end(group));
+    if (group < 0 || group > regexp._groupCount) {
+      throw new IndexOutOfRangeException(group);
+    }
+    int startIndex = _start(group);
+    int endIndex = _end(group);
+    if (startIndex == -1) {
+      assert(endIndex == -1);
+      return null;
+    }
+    return str.substringUnchecked_(startIndex, endIndex);
   }
 
   String operator [](int group) {
-    return str.substringUnchecked_(_start(group), _end(group));
+    return this.group(group);
   }
 
   List<String> groups(List<int> groups) {
     var groupsList = new List<String>(groups.length);
     for (int i = 0; i < groups.length; i++) {
-      int grp_idx = groups[i];
-      groupsList[i] = str.substringUnchecked_(_start(grp_idx), _end(grp_idx));
+      groupsList[i] = group(groups[i]);
     }
     return groupsList;
   }
@@ -45,7 +53,7 @@ class JSRegExpMatch implements Match {
   final RegExp regexp;
   final String str;
   final List<int> _match;
-  static final int _kMatchPair = 2;
+  static final int MATCH_PAIR = 2;
 }