Remove deprecated Strings class.

tests/standalone/io/secure_socket_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 35 matching lines @@
   }
   useReadList = () {
     var buffer = new List.fixedLength(2000);
     int len = secure.readList(buffer, 0, 2000);
     var received = new String.fromCharCodes(buffer.getRange(0, len));
     chunks.add(received);
     secure.onData = useRead;
   };
   secure.onData = useRead;
   secure.onClosed = () {
-    String fullPage = Strings.concatAll(chunks);
+    String fullPage = chunks.join();
     Expect.isTrue(fullPage.contains('</body></html>'));
     keepAlive.close();
   };
 }