Make StringBase._substringUnchecked a native function.

runtime/lib/string.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.
 
 /**
  * [StringBase] contains common methods used by concrete String implementations,
  * e.g., OneByteString.
  */
 class StringBase {
 
@@ skipping 134 matching lines @@
 
     if ((startIndex < 0) || (startIndex > this.length)) {
       throw new IndexOutOfRangeException(startIndex);
     }
     if ((endIndex < 0) || (endIndex > this.length)) {
       throw new IndexOutOfRangeException(endIndex);
     }
     if (startIndex > endIndex) {
       throw new IndexOutOfRangeException(startIndex);
     }
-    return substringUnchecked_(startIndex, endIndex);
+    return _substringUnchecked(startIndex, endIndex);
   }
 
-  String substringUnchecked_(int startIndex, int endIndex) {
-    int len = endIndex - startIndex;
-    List<int> charCodes = new List<int>(len);
-    for (int i = 0; i < len; i++) {
-      charCodes[i] = this.charCodeAt(startIndex + i);
-    }
-    return StringBase.createFromCharCodes(charCodes);
-  }
+  String _substringUnchecked(int startIndex, int endIndex)
+      native "StringBase_substringUnchecked";
 
   String trim() {
     final int len = this.length;
     int first = 0;
     for (; first < len; first++) {
       if (!_isWhitespace(this.charCodeAt(first))) {
         break;
       }
     }
     if (len == first) {
       // String contains only whitespaces.
       return "";
     }
     int last = len - 1;
     for (; last >= first; last--) {
       if (!_isWhitespace(this.charCodeAt(last))) {
         break;
       }
     }
     if ((first == 0) && (last == (len - 1))) {
       // Returns this string if it does not have leading or trailing
       // whitespaces.
       return this;
     } else {
-      return substringUnchecked_(first, last + 1);
+      return _substringUnchecked(first, last + 1);
     }
   }
 
   bool contains(Pattern pattern, [int startIndex = 0]) {
     if (pattern is String) {
       return indexOf(pattern, startIndex) >= 0;
     }
     return pattern.allMatches(this.substring(startIndex)).iterator().hasNext();
   }
 
@@ skipping 296 matching lines @@
     for (int g in groups) {
       result.add(group(g));
     }
     return result;
   }
 
   final int _start;
   final String str;
   final String pattern;
 }