Fix DevFS writes with subdirectories

runtime/observatory/tests/service/dev_fs_test.dart

 // Copyright (c) 2016, 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.
 // VMOptions=--error_on_bad_type --error_on_bad_override
 
 import 'dart:convert';
 import 'package:observatory/service_io.dart';
 import 'package:unittest/unittest.dart';
 import 'test_helper.dart';
 
@@ skipping 40 matching lines @@
     } on ServerRpcException catch(e) {
       caughtException = true;
       expect(e.code, equals(ServerRpcException.kFileSystemDoesNotExist));
       expect(e.message, "_deleteDevFS: file system 'alpha' does not exist");
     }
     expect(caughtException, isTrue);
   },
 
   (VM vm) async {
     var fsId = 'banana';
-    var filePath = '/foobar.dat';
+    var filePath = '/foo/bar.dat';
     var fileContents = BASE64.encode(UTF8.encode('fileContents'));
 
     var result;
     // Create DevFS.
     result = await vm.invokeRpcNoUpgrade('_createDevFS', { 'fsName': fsId });
     expect(result['type'], equals('FileSystem'));
     expect(result['name'], equals(fsId));
     expect(result['uri'], new isInstanceOf<String>());
 
     bool caughtException = false;
@@ skipping 77 matching lines @@
 
     // Delete DevFS.
     result = await vm.invokeRpcNoUpgrade('_deleteDevFS', {
         'fsName': fsId,
     });
     expect(result['type'], equals('Success'));
   },
 ];
 
 main(args) async => runVMTests(args, tests);