Make StringBase._substringUnchecked a native function.

runtime/lib/regexp_patch.dart

 // Copyright (c) 2012, 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);
   }
@@ skipping 13 matching lines @@
   String group(int groupIdx) {
     if (groupIdx < 0 || groupIdx > regexp._groupCount) {
       throw new IndexOutOfRangeException(groupIdx);
     }
     int startIndex = _start(groupIdx);
     int endIndex = _end(groupIdx);
     if (startIndex == -1) {
       assert(endIndex == -1);
       return null;
     }
-    return str.substringUnchecked_(startIndex, endIndex);
+    return str._substringUnchecked(startIndex, endIndex);
   }
 
   String operator [](int groupIdx) {
     return this.group(groupIdx);
   }
 
   List<String> groups(List<int> groupsSpec) {
     var groupsList = new List<String>(groupsSpec.length);
     for (int i = 0; i < groupsSpec.length; i++) {
       groupsList[i] = group(groupsSpec[i]);
@@ skipping 53 matching lines @@
   /* patch */ bool hasMatch(String str) {
     List match = _ExecuteMatch(str, 0);
     return (match === null) ? false : true;
   }
 
   /* patch */ String stringMatch(String str) {
     List match = _ExecuteMatch(str, 0);
     if (match === null) {
       return null;
     }
-    return str.substringUnchecked_(match[0], match[1]);
+    return str._substringUnchecked(match[0], match[1]);
   }
 
   /* patch */ String get pattern() native "JSSyntaxRegExp_getPattern";
 
   /* patch */ bool get multiLine() native "JSSyntaxRegExp_multiLine";
 
   /* patch */ bool get ignoreCase() native "JSSyntaxRegExp_ignoreCase";
 
   int get _groupCount() native "JSSyntaxRegExp_getGroupCount";
 
   List _ExecuteMatch(String str, int start_index)
       native "JSSyntaxRegExp_ExecuteMatch";
 }