Remove deprecated Strings class.

tests/standalone/io/secure_stream_test.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.
 //
 // VMOptions=
 // VMOptions=--short_socket_read
 // The --short_socket_write option does not work with external server
 // www.google.dk.  Add this to the test when we have secure server sockets.
 // See TODO below.
 
@@ skipping 11 matching lines @@
   List<String> chunks = <String>[];
   var input = secure.inputStream;
   var output = secure.outputStream;
 
   output.write("GET / HTTP/1.0\r\nHost: www.google.dk\r\n\r\n".charCodes);
   output.close();
   input.onData = () {
     chunks.add(new String.fromCharCodes(input.read()));
   };
   input.onClosed = () {
-    String fullPage = Strings.concatAll(chunks);
+    String fullPage = chunks.join();
     Expect.isTrue(fullPage.contains('</body></html>'));
     keepAlive.close();
   };
 }