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

pkg/front_end/lib/memory_file_system.dart

Index: pkg/front_end/lib/memory_file_system.dart
diff --git a/pkg/front_end/lib/memory_file_system.dart b/pkg/front_end/lib/memory_file_system.dart
index e7382adfb38a38d51b534894d9c7ce6de27d1a25..6c57167a87d45e1620fadab02a82fe585658256d 100644
--- a/pkg/front_end/lib/memory_file_system.dart
+++ b/pkg/front_end/lib/memory_file_system.dart
@@ -8,8 +8,6 @@ import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
-import 'package:path/path.dart' as p;
-
 import 'file_system.dart';
 
 /// Concrete implementation of [FileSystem] which performs its operations on an
@@ -17,10 +15,13 @@ import 'file_system.dart';
 ///
 /// Not intended to be implemented or extended by clients.
 class MemoryFileSystem implements FileSystem {
-  @override
-  final p.Context context;
-
   final Map<Uri, Uint8List> _files = {};
+  final Map<Uri, DateTime> _lastModified = {};
+
+  // Counter used to create mock last-modification timestamps. The memory
+  // file-system is mainly used for testing, so we use mock timestamps to avoid
+  // introducing non-determinism.
+  int _lastUpdate = 0;
 
   /// The "current directory" in the in-memory virtual file system.
   ///
@@ -29,7 +30,7 @@ class MemoryFileSystem implements FileSystem {
   /// Always ends in a trailing '/'.
   Uri currentDirectory;
 
-  MemoryFileSystem(this.context, Uri currentDirectory)
+  MemoryFileSystem(Uri currentDirectory)
       : currentDirectory = _addTrailingSlash(currentDirectory);
 
   @override
@@ -80,12 +81,24 @@ class MemoryFileSystemEntity implements FileSystemEntity {
     return UTF8.decode(contents);
   }
 
+  @override
+  Future<bool> exists() async => _fileSystem._files[uri] != null;
+
+  @override
+  Future<DateTime> lastModified() async {
+    var lastModified = _fileSystem._lastModified[uri];
+    if (lastModified == null) {
+      throw new Exception('File does not exist');
+    }
+    return lastModified;
+  }
+
   /// Writes the given raw bytes to this file system entity.
   ///
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsBytesSync(List<int> bytes) {
-    _fileSystem._files[uri] = new Uint8List.fromList(bytes);
+    _update(uri, new Uint8List.fromList(bytes));
   }
 
   /// Writes the given string to this file system entity.
@@ -98,6 +111,12 @@ class MemoryFileSystemEntity implements FileSystemEntity {
     // Note: the return type of UTF8.encode is List<int>, but in practice it
     // always returns Uint8List.  We rely on that for efficiency, so that we
     // don't have to make an extra copy.
-    _fileSystem._files[uri] = UTF8.encode(s) as Uint8List;
+    _update(uri, UTF8.encode(s) as Uint8List);
+  }
+
+  void _update(Uri uri, Uint8List data) {
+    _fileSystem._files[uri] = data;
+    _fileSystem._lastModified[uri] =
+        new DateTime.fromMicrosecondsSinceEpoch(++_fileSystem._lastUpdate);
   }
 }