Remove deprecated Strings class.

tests/standalone/io/secure_socket_bad_certificate_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 45 matching lines @@
     Expect.isTrue(certificate.startValidity < new DateTime.now());
     Expect.isTrue(certificate.endValidity > new DateTime.now());
     return acceptCertificate;
   };
   secure.onData = () {
     Expect.isTrue(acceptCertificate);
     chunks.add(new String.fromCharCodes(secure.read()));
   };
   secure.onClosed = () {
     Expect.isTrue(acceptCertificate);
-    String fullPage = Strings.concatAll(chunks);
+    String fullPage = chunks.join();
     Expect.isTrue(fullPage.contains('</body></html>'));
     completer.complete(null);
   };
   secure.onError = (e) {
     Expect.isFalse(acceptCertificate);
     completer.complete(null);
   };
   return completer.future;
 }