Remove deprecated Strings class.

utils/lib/file_system.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.
 
 // TODO(terry): Investigate common library for file I/O shared between frog and tools.
 
 /** Abstraction for file systems and utility functions to manipulate paths. */
 library file_system;
 
 /**
@@ skipping 29 matching lines @@
     if (piece == '..' && pieces.length > 0 && pieces.last != '.'
       && pieces.last != '..') {
       pieces.removeLast();
     } else if (piece != '') {
       if (pieces.length > 0 && pieces.last == '.') {
         pieces.removeLast();
       }
       pieces.add(piece);
     }
   }
-  return Strings.join(pieces, '/');
+  return pieces.join('/');
 }
 
 /** Returns the directory name for the [path]. */
 String dirname(String path) {
   path = canonicalizePath(path);
 
   int lastSlash = path.lastIndexOf('/', path.length);
   if (lastSlash == -1) {
     return '.';
   } else {
     return path.substring(0, lastSlash);
   }
 }
 
 /** Returns the file name without directory for the [path]. */
 String basename(String path) {
   path = canonicalizePath(path);
 
   int lastSlash = path.lastIndexOf('/', path.length);
   if (lastSlash == -1) {
     return path;
   } else {
     return path.substring(lastSlash + 1);
   }
 }