Reapply conversion of most core interfaces to abstract classes.

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