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 6e57c266c067bdd9b731335abc37ece3bfa672ec..0afb39e7a7b72fd7fc812b50da4879f82ffb4364 100644
--- a/pkg/front_end/lib/file_system.dart
+++ b/pkg/front_end/lib/file_system.dart
@@ -6,8 +6,6 @@ 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;
@@ -17,12 +15,6 @@ import 'package:path/path.dart' as path;
 ///
 /// 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;

[Siggi Cherem (dart-lang), 2017/04/27 00:08:50]

I can split this change if you prefer, but very soon will be gone one way or
another. Because I did it this early I had to update the kernel_generator to
directly use uri.toFilePath(), which is not perfect. I'll be changing
kernel_generator in the next couple days to make it use fasta anyways, so I'm ok
with that temporary change. 

-
   /// Returns a [FileSystemEntity] corresponding to the given [uri].
   ///
   /// Uses of `..` and `.` in the URI are normalized before returning.
@@ -41,8 +33,7 @@ abstract class FileSystem {
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystemEntity {
-  /// Returns the absolute normalized URI represented by this file system
-  /// entity.
+  /// 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.
@@ -65,4 +56,12 @@ 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();
 }