Fix more strong mode errors in analyzer

pkg/analyzer/lib/file_system/memory_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.memory_file_system;
 
 import 'dart:async';
 import 'dart:collection';
 import 'dart:core' hide Resource;
 
@@ skipping 278 matching lines @@
   List<int> readAsBytesSync() {
     throw new FileSystemException(path, 'File could not be read');
   }
 
   @override
   String readAsStringSync() {
     throw new FileSystemException(path, 'File could not be read');
   }
 
   @override
-  Resource renameSync(String newPath) {
+  File renameSync(String newPath) {
     throw new FileSystemException(path, 'File could not be renamed');
   }
 
   @override
   void writeAsBytesSync(List<int> bytes) {
     throw new FileSystemException(path, 'File could not be written');
   }
 }
 
 /**
@@ skipping 260 matching lines @@
   bool operator ==(other) {
     if (runtimeType != other.runtimeType) {
       return false;
     }
     return path == other.path;
   }
 
   @override
   String toString() => path;
 }