Fix performance degradation in string buffer benchmarks: concat in native instead of in Dart.

runtime/lib/string_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.
 
 patch class String {
   /* patch */ factory String.fromCharCodes(Iterable<int> charCodes) {
     return _StringBase.createFromCharCodes(charCodes);
   }
 }
 
@@ skipping 491 matching lines @@
 
   List<int> get codeUnits => new _CodeUnits(this);
 
   Runes get runes => new Runes(this);
 
   String toUpperCase() native "String_toUpperCase";
 
   String toLowerCase() native "String_toLowerCase";
 
   // Concatenate ['start', 'end'[ elements of 'strings'. 'strings' must contain
-  // String elements. Optimized for OneByteStrings.
+  // String elements. TODO(srdjan): optimize it.
   static String _concatRange(List<String> strings, int start, int end) {
     if ((end - start) == 1) {
       return strings[start];
     }
-    final numValues = strings.length;
-    if ((start == 0) && (end == numValues)) {
-      int totalLength = 0;
-      for (int i = 0; i < numValues; i++) {
-        String s = strings[i];
-        if (s._cid != _OneByteString._classId) {
-          return _concatRangeNative(strings, start, end);
-        }
-        totalLength += s.length;
-      }
-      return _OneByteString._concatAll(strings, totalLength);
-    }
     return _concatRangeNative(strings, start, end);
   }
 
   // Call this method if not all list elements are known to be OneByteString(s).
   // 'strings' must be an _List or _GrowableList.
   static String _concatRangeNative(List<String> strings, int start, int end)
       native "String_concatRange";
 }
 
 
@@ skipping 159 matching lines @@
 class _CodeUnits extends Object with ListMixin<int>,
                                      UnmodifiableListMixin<int> {
   /** The string that this is the code units of. */
   String _string;
 
   _CodeUnits(this._string);
 
   int get length => _string.length;
   int operator[](int i) => _string.codeUnitAt(i);
 }