Remove deprecated Strings class.

tests/language/string_join_test.dart

 // Copyright (c) 2011, 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.
 // Regression test ensuring that only ObjectArrays are handed to the VM code.
 
 class StringJoinTest {
   static testMain() {
     List<String> ga = new List<String>();
     ga.add("a");
     ga.add("b");
-    Expect.equals("ab", Strings.join(ga, ""));
+    Expect.equals("ab", ga.join());
+    Expect.equals("ab", ga.join(""));
   }
 }
 
 main() {
   StringJoinTest.testMain();
 }