Remove deprecated Strings class.

runtime/lib/string_buffer_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 StringBuffer {
   List<String> _buffer;
   int _length;
 
   /// Creates the string buffer with an initial content.
   /* patch */ StringBuffer([Object content = ""]) {
@@ skipping 25 matching lines @@
   /// Clears the string buffer.
   /* patch */ void clear() {
     _buffer = new List<String>();
     _length = 0;
   }
 
   /// Returns the contents of buffer as a concatenated string.
   /* patch */ String toString() {
     if (_buffer.length == 0) return "";
     if (_buffer.length == 1) return _buffer[0];
-    String result = Strings.concatAll(_buffer);
+    String result = _StringBase.concatAll(_buffer);
     _buffer.clear();
     _buffer.add(result);
     // Since we track the length at each add operation, there is no
     // need to update it in this function.
     return result;
   }
 }