Add argument check to Directory and Link constructors

sdk/lib/io/link.dart

 // Copyright (c) 2013, 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.
 
 part of dart.io;
 
 /**
  * [Link] objects are references to filesystem links.
  *
  */
@@ skipping 112 matching lines @@
    */
   String targetSync();
 }
 
 
 class _Link extends FileSystemEntity implements Link {
   final String path;
 
   SendPort _fileService;
 
-  _Link(String this.path);
+  _Link(String this.path) {
+    if (path is! String) {
+      throw new ArgumentError('${Error.safeToString(path)} '
+                              'is not a String');
+    }
+  }
+
 
   _Link.fromPath(Path inputPath) : path = inputPath.toNativePath();
 
   String toString() => "Link: '$path'";
 
   Future<bool> exists() => FileSystemEntity.isLink(path);
 
   bool existsSync() => FileSystemEntity.isLinkSync(path);
 
   Future<FileStat> stat() => FileStat.stat(path);
@@ skipping 165 matching lines @@
       if (path != null) {
         sb.write(", path = $path");
       }
     }
     return sb.toString();
   }
   final String message;
   final String path;
   final OSError osError;
 }