Fix analyzer tests in checked mode.

utils/kernel-service/kernel-service.dart

Index: utils/kernel-service/kernel-service.dart
diff --git a/utils/kernel-service/kernel-service.dart b/utils/kernel-service/kernel-service.dart
index ec7e2aa2ae1a022c1b83d3433b516790f3980d4d..dc7dacabe82cd00697b06f50019202a0061e7eb2 100644
--- a/utils/kernel-service/kernel-service.dart
+++ b/utils/kernel-service/kernel-service.dart
@@ -24,6 +24,7 @@ import 'dart:async';
 import 'dart:io';
 import 'dart:isolate';
 
+import 'package:front_end/file_system.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/physical_file_system.dart';
 import 'package:front_end/src/fasta/vm.dart'
@@ -100,8 +101,8 @@ Future _processLoadRequest(request) async {
 // raw file content Uint8List this function builds up and returns
 // MemoryFileSystem instance that can be used instead of
 // PhysicalFileSystem.instance by the frontend.
-FileSystem _buildMemoryFileSystem(List namedSources) {
-  FileSystem fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
+MemoryFileSystem _buildMemoryFileSystem(List namedSources) {
+  MemoryFileSystem fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
   for (int i = 0; i < namedSources.length ~/ 2; i++) {
     fileSystem
         .entityForUri(Uri.parse(namedSources[i * 2]))