Fix strong mode errors in dart:io.

sdk/lib/io/directory_impl.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.
 
 part of dart.io;
 
 class _Directory extends FileSystemEntity implements Directory {
   final String path;
 
   _Directory(this.path) {
     if (path is! String) {
       throw new ArgumentError('${Error.safeToString(path)} '
                               'is not a String');
     }
   }
 
   external static _current();
   external static _setCurrent(path);
   external static _createTemp(String path);
   external static String _systemTemp();
   external static _exists(String path);
   external static _create(String path);
   external static _deleteNative(String path, bool recursive);
   external static _rename(String path, String newPath);
-  external static List _list(String path, bool recursive, bool followLinks);
+  external static void _fillWithDirectoryListing(
+      List<FileSystemEntity> list, String path, bool recursive,
+      bool followLinks);
 
   static Directory get current {
     var result = _current();
     if (result is OSError) {
       throw new FileSystemException(
           "Getting current working directory failed", "", result);
     }
     return new _Directory(result);
   }
 
@@ skipping 179 matching lines @@
   }
 
   Stream<FileSystemEntity> list({bool recursive: false,
                                  bool followLinks: true}) {
     return new _AsyncDirectoryLister(
         FileSystemEntity._ensureTrailingPathSeparators(path),
         recursive,
         followLinks).stream;
   }
 
-  List listSync({bool recursive: false, bool followLinks: true}) {
+  List<FileSystemEntity> listSync(
+      {bool recursive: false, bool followLinks: true}) {
     if (recursive is! bool || followLinks is! bool) {
       throw new ArgumentError();
     }
-    return _list(
+    var result = <FileSystemEntity>[];
+    _fillWithDirectoryListing(
+        result,
         FileSystemEntity._ensureTrailingPathSeparators(path),
         recursive,
         followLinks);
+    return result;
   }
 
   String toString() => "Directory: '$path'";
 
   bool _isErrorResponse(response) =>
       response is List && response[0] != _SUCCESS_RESPONSE;
 
   _exceptionOrErrorFromResponse(response, String message) {
     assert(_isErrorResponse(response));
     switch (response[_ERROR_RESPONSE_ERROR_TYPE]) {
@@ skipping 170 matching lines @@
       controller.addError(
           new FileSystemException("Directory listing failed",
                                   errorPath,
                                   err));
     } else {
       controller.addError(
           new FileSystemException("Internal error"));
     }
   }
 }