Fix more strong mode errors in analyzer

pkg/analyzer/lib/file_system/physical_file_system.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.file_system.physical_file_system;
 
 import 'dart:async';
 import 'dart:core' hide Resource;
 import 'dart:io' as io;
 
@@ skipping 114 matching lines @@
   String readAsStringSync() {
     try {
       io.File file = _entry as io.File;
       return FileBasedSource.fileReadMode(file.readAsStringSync());
     } on io.FileSystemException catch (exception) {
       throw new FileSystemException(exception.path, exception.message);
     }
   }
 
   @override
-  Resource renameSync(String newPath) {
+  File renameSync(String newPath) {
     try {
       io.File file = _entry as io.File;
       io.File newFile = file.renameSync(newPath);
       return new _PhysicalFile(newFile);
     } on io.FileSystemException catch (exception) {
       throw new FileSystemException(exception.path, exception.message);
     }
   }
 
   @override
@@ skipping 105 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }