Extend file-system abstraction with a couple methods, remove context

pkg/front_end/test/incremental_resolved_ast_generator_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:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/standard_resolution_map.dart';
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/src/dart/sdk/sdk.dart';
 import 'package:front_end/compiler_options.dart';
 import 'package:front_end/incremental_resolved_ast_generator.dart';
 import 'package:front_end/memory_file_system.dart';
-import 'package:path/path.dart' as pathos;
 import 'package:test/test.dart';
 import 'package:test_reflective_loader/test_reflective_loader.dart';
 
 main() {
   defineReflectiveSuite(() {
     defineReflectiveTests(IncrementalResolvedAstGeneratorTest);
   });
 }
 
 final _sdkSummary = _readSdkSummary();
 
 List<int> _readSdkSummary() {
   var resourceProvider = PhysicalResourceProvider.INSTANCE;
   var sdk = new FolderBasedDartSdk(resourceProvider,
       FolderBasedDartSdk.defaultSdkDirectory(resourceProvider))
     ..useSummary = true;
   var path = resourceProvider.pathContext
       .join(sdk.directory.path, 'lib', '_internal', 'strong.sum');
   return resourceProvider.getFile(path).readAsBytesSync();
 }
 
 @reflectiveTest
 class IncrementalResolvedAstGeneratorTest {
   static final sdkSummaryUri = Uri.parse('special:sdk_summary');
 
   /// Virtual filesystem for testing.
-  final fileSystem = new MemoryFileSystem(pathos.posix, Uri.parse('file:///'));
+  final fileSystem = new MemoryFileSystem(Uri.parse('file:///'));
 
   /// The object under test.
   IncrementalResolvedAstGenerator incrementalResolvedAstGenerator;
 
   Future<Map<Uri, Map<Uri, CompilationUnit>>> getInitialProgram(
       Uri startingUri) async {
     fileSystem.entityForUri(sdkSummaryUri).writeAsBytesSync(_sdkSummary);
     incrementalResolvedAstGenerator = new IncrementalResolvedAstGenerator(
         startingUri,
         new CompilerOptions()
@@ skipping 111 matching lines @@
     }
     throw fail('No function declaration found with name "$name"');
   }
 
   NodeList<Statement> _getFunctionStatements(FunctionDeclaration function) {
     var body = function.functionExpression.body;
     expect(body, new isInstanceOf<BlockFunctionBody>());
     return (body as BlockFunctionBody).block.statements;
   }
 }