Make failed capturing parenthesis produce null instead of "".

runtime/lib/regexp.dart

 // Copyright (c) 2011, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 class JSRegExpMatch implements Match {
   JSRegExpMatch(this.regexp, this.str, this._match);
 
   int start() {
     return _start(0);
   }
 
   int end() {
     return _end(0);
   }
 
   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;
   }
 
   int groupCount() {
     return regexp._groupCount;
   }
 
   final RegExp regexp;
   final String str;
   final List<int> _match;
-  static final int _kMatchPair = 2;
+  static final int MATCH_PAIR = 2;
 }
 
 
 class JSSyntaxRegExp implements RegExp {
   const factory JSSyntaxRegExp(
       String pattern,
       [bool multiLine = false,
        bool ignoreCase = false]) native "JSSyntaxRegExp_factory";
 
   Match firstMatch(String str) {
@@ skipping 37 matching lines @@
 
   bool get multiLine() native "JSSyntaxRegExp_multiLine";
 
   bool get ignoreCase() native "JSSyntaxRegExp_ignoreCase";
 
   int get _groupCount() native "JSSyntaxRegExp_getGroupCount";
 
   List _ExecuteMatch(String str, int start_index)
       native "JSSyntaxRegExp_ExecuteMatch";
 }