Convert String to a class.

sdk/lib/core/strings.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.
 
 abstract class Strings {
   /**
    * Joins all the given strings to create a new string.
    */
-  static String join(List<String> strings, String separator) {
-    return _StringImpl.join(strings, separator);
-  }
+  external static String join(List<String> strings, String separator);
 
   /**
    * Concatenates all the given strings to create a new string.
    */
-  static String concatAll(List<String> strings) {
-    return _StringImpl.concatAll(strings);
-  }
+  external static String concatAll(List<String> strings);
 }