Replace fake directories with actual createDirectory().

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

 // Copyright (c) 2017, 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.
 
 import 'dart:async';
 
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/memory_file_system.dart';
 import 'package:front_end/src/base/processed_options.dart';
 import 'package:front_end/src/fasta/fasta.dart' show ByteSink;
 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';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(ProcessedOptionsTest);
   });
 }
 
 @reflectiveTest
@@ skipping 110 matching lines @@
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()
       ..fileSystem = fileSystem
       ..packagesFileUri = Uri.parse('file:///explicit.packages');
     var processed = new ProcessedOptions(raw);
     var uriTranslator = await processed.getUriTranslator();
     checkPackageExpansion('foo', 'baz', uriTranslator.packages);
   }
 
   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'))
         .writeAsStringSync('foo:bar\n');
     // This one should be used.
     fileSystem
         .entityForUri(Uri.parse('file:///base/location/.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()..fileSystem = fileSystem;
     var processed = new ProcessedOptions(
         raw, false, [Uri.parse('file:///base/location/script.dart')]);
     var uriTranslator = await processed.getUriTranslator();
     checkPackageExpansion('foo', 'base/location/baz', uriTranslator.packages);
   }
 
   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'))
         .writeAsStringSync('foo:bar\n');
     // This one should be used.
     fileSystem
         .entityForUri(Uri.parse('file:///base/.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()..fileSystem = fileSystem;
     var processed = new ProcessedOptions(
         raw, false, [Uri.parse('file:///base/location/script.dart')]);
     var uriTranslator = await processed.getUriTranslator();
     checkPackageExpansion('foo', 'base/baz', uriTranslator.packages);
   }
 
   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
         .entityForUri(Uri.parse('file:///.packages'))
         .writeAsStringSync('foo:bar\n');
     fileSystem
         .entityForUri(Uri.parse('file:///base/.packages'))
         .writeAsStringSync('foo:baz\n');
     var raw = new CompilerOptions()..fileSystem = fileSystem;
     var processed = new ProcessedOptions(
         raw, false, [Uri.parse('file:///base/location/script.dart')]);
     var uriTranslator = await processed.getUriTranslator();
     checkPackageExpansion(
         'foo', 'base/location/packages/foo', uriTranslator.packages);
   }
 
   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()
       ..fileSystem = fileSystem
       ..onError = (e) => errors.add(e);
     var processed = new ProcessedOptions(
         raw, false, [Uri.parse('file:///base/location/script.dart')]);
     var uriTranslator = await processed.getUriTranslator();
     expect(errors, isEmpty);
     expect(uriTranslator.packages.asMap(), isEmpty);
@@ skipping 163 matching lines @@
     var index = messageTemplate.indexOf('#');
     var prefix = messageTemplate.substring(0, index - 1);
 
     // Check that the prefix is not empty and that it contains more than one
     // word.
     expect(prefix.length > 0, isTrue);
     expect(prefix.contains(' '), isTrue);
     return prefix;
   }
 }