Fix analyzer tests in checked mode.

pkg/front_end/lib/src/fasta/vm.dart

 // Copyright (c) 2017, 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.
 
 /// API for compiling Dart source code to .dill (Kernel IR) files.
 library front_end.vm;
 // TODO(ahe): Convert this file to use the API in `../../kernel_generator.dart`
 // and `../../compiler_options.dart`.
 
 import 'dart:async' show Future;
 
 import 'dart:io' show File, Platform;
 
 import 'dart:typed_data' show Uint8List;
 
+import 'package:front_end/file_system.dart';
 import 'package:front_end/physical_file_system.dart';
 
 import 'fasta.dart' as fasta;
 
 /// Compilation status codes.
 ///
 /// Note: The [index] property of these constants must match
 /// `Dart_KernelCompilationStatus` in
 /// [dart_api.h](../../../../runtime/include/dart_api.h).
 enum Status {
@@ skipping 126 matching lines @@
     if (await fileSystem.entityForUri(packagesFile).exists()) {
       return packagesFile;
     }
     if (dir.parent.path == dir.path) {
       break;
     }
     dir = dir.parent;
   }
   return null;
 }