MemoryResourceProvider.getFolder() should not create the folder.

pkg/analyzer/test/src/context/builder_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.
 
 library analyzer.test.src.context.context_builder_test;
 
 import 'package:analyzer/file_system/file_system.dart';
 import 'package:analyzer/file_system/memory_file_system.dart';
 import 'package:analyzer/plugin/options.dart';
 import 'package:analyzer/source/package_map_resolver.dart';
@@ skipping 250 matching lines @@
     Packages packages = builder.createPackageMap(projectPath);
     expect(packages, isNotNull);
     Map<String, Uri> map = packages.asMap();
     expect(map, hasLength(2));
     expect(map['foo'], fooUri);
     expect(map['bar'], barUri);
   }
 
   void test_createPackageMap_none() {
     String rootPath = resourceProvider.convertPath('/root');
+    resourceProvider.newFolder(rootPath);
     Packages packages = builder.createPackageMap(rootPath);
     expect(packages, same(Packages.noPackages));
   }
 
   void test_createSourceFactory_fileProvider() {
     String rootPath = resourceProvider.convertPath('/root');
     Folder rootFolder = resourceProvider.getFolder(rootPath);
     createDefaultSdk(rootFolder);
     String projectPath = pathContext.join(rootPath, 'project');
     String packageFilePath = pathContext.join(projectPath, '.packages');
@@ skipping 450 matching lines @@
       fail('Unexpected invocation of optionsProcessed');
     }
     expect(options, hasLength(expectedOptions.length));
     for (String key in expectedOptions.keys) {
       expect(options.containsKey(key), isTrue, reason: 'missing key $key');
       expect(options[key], expectedOptions[key],
           reason: 'values for key $key do not match');
     }
   }
 }