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

pkg/front_end/lib/file_system.dart

Index: pkg/front_end/lib/file_system.dart
diff --git a/pkg/front_end/lib/file_system.dart b/pkg/front_end/lib/file_system.dart
index 0afb39e7a7b72fd7fc812b50da4879f82ffb4364..6e57c266c067bdd9b731335abc37ece3bfa672ec 100644
--- a/pkg/front_end/lib/file_system.dart
+++ b/pkg/front_end/lib/file_system.dart
@@ -6,6 +6,8 @@ library front_end.file_system;
 
 import 'dart:async';
 
+import 'package:path/path.dart' as path;
+
 /// Abstract interface to file system operations.
 ///
 /// All front end interaction with the file system goes through this interface;
@@ -15,6 +17,12 @@ import 'dart:async';
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystem {
+  /// Returns a path context suitable for use with this [FileSystem].
+  ///
+  /// TODO(paulberry): try to eliminate all usages of this.  Since the
+  /// FileSystem API now uses URIs rather than paths, it should not be needed.
+  path.Context get context;
+
   /// Returns a [FileSystemEntity] corresponding to the given [uri].
   ///
   /// Uses of `..` and `.` in the URI are normalized before returning.
@@ -33,7 +41,8 @@ abstract class FileSystem {
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystemEntity {
-  /// The absolute normalized URI represented by this file system entity.
+  /// Returns the absolute normalized URI represented by this file system
+  /// entity.
   ///
   /// Note: this is not necessarily the same as the URI that was passed to
   /// [FileSystem.entityForUri], since the URI might have been normalized.
@@ -56,12 +65,4 @@ abstract class FileSystemEntity {
   /// file exists, the entity is a directory, or the file is not valid UTF-8),
   /// the future is completed with an [Exception].
   Future<String> readAsString();
-
-  /// Whether this file system entity exists.
-  Future<bool> exists();
-
-  /// Extracts the last-modification time of the file system entity, if it
-  /// exists and it is a file, otherwise the future is completed with an
-  /// [Exception].
-  Future<DateTime> lastModified();
 }