Replace fake directories with actual createDirectory().

pkg/front_end/test/src/base/processed_options_test.dart

Index: pkg/front_end/test/src/base/processed_options_test.dart
diff --git a/pkg/front_end/test/src/base/processed_options_test.dart b/pkg/front_end/test/src/base/processed_options_test.dart
index 883dbc995586146497aa3df2e15b6d110a9f38a6..2f6bd65496c4dd2e13007317e14de7c6ae88f3ae 100644
--- a/pkg/front_end/test/src/base/processed_options_test.dart
+++ b/pkg/front_end/test/src/base/processed_options_test.dart
@@ -12,7 +12,6 @@ import 'package:front_end/src/fasta/fasta_codes.dart';
 import 'package:kernel/binary/ast_to_binary.dart' show BinaryPrinter;
 import 'package:kernel/kernel.dart' show Program, Library, CanonicalName;
 import 'package:package_config/packages.dart' show Packages;
-
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
@@ -143,14 +142,14 @@ class ProcessedOptionsTest {
   }
 
   test_getUriTranslator_implicitPackagesFile_nextToScript() async {
-    // Fake the existence of the base directory.
+    // Create the base directory.
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/'))
-        .writeAsStringSync('');
+        .createDirectory();
     // Packages directory should be ignored (.packages file takes precedence).
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/packages/'))
-        .writeAsStringSync('');
+        .createDirectory();
     // This .packages file should be ignored.
     fileSystem
         .entityForUri(Uri.parse('file:///.packages'))
@@ -167,10 +166,10 @@ class ProcessedOptionsTest {
   }
 
   test_getUriTranslator_implicitPackagesFile_searchAbove() async {
-    // Fake the existence of the base directory.
+    // Create the base directory.
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/'))
-        .writeAsStringSync('');
+        .createDirectory();
     // This .packages file should be ignored.
     fileSystem
         .entityForUri(Uri.parse('file:///.packages'))
@@ -187,13 +186,13 @@ class ProcessedOptionsTest {
   }
 
   test_getUriTranslator_implicitPackagesFile_packagesDirectory() async {
-    // Fake the existence of the base directory.
+    // Create the base directory.
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/'))
-        .writeAsStringSync('');
+        .createDirectory();
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/packages/'))
-        .writeAsStringSync('');
+        .createDirectory();
 
     // Both of these .packages file should be ignored.
     fileSystem
@@ -211,10 +210,10 @@ class ProcessedOptionsTest {
   }
 
   test_getUriTranslator_implicitPackagesFile_noPackages() async {
-    // Fake the existence of the base directory.
+    // Create the base directory.
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/'))
-        .writeAsStringSync('');
+        .createDirectory();
     var errors = [];
     // .packages file should be ignored.
     var raw = new CompilerOptions()