MemoryResourceProvider.getFolder() should not create the folder.

pkg/analyzer/test/src/context/builder_test.dart

Index: pkg/analyzer/test/src/context/builder_test.dart
diff --git a/pkg/analyzer/test/src/context/builder_test.dart b/pkg/analyzer/test/src/context/builder_test.dart
index 67a3a375a9e6b774ef24ad58d24a0c6fc28c996c..b63b83d1ca82fcaf5a268b4cbaa4a8a806ee44de 100644
--- a/pkg/analyzer/test/src/context/builder_test.dart
+++ b/pkg/analyzer/test/src/context/builder_test.dart
@@ -268,6 +268,7 @@ bar:$barUri
 
   void test_createPackageMap_none() {
     String rootPath = resourceProvider.convertPath('/root');
+    resourceProvider.newFolder(rootPath);
     Packages packages = builder.createPackageMap(rootPath);
     expect(packages, same(Packages.noPackages));
   }