First cut at a file system abstraction for the front end.

pkg/front_end/lib/file_system.dart

Index: pkg/front_end/lib/file_system.dart
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+// Copyright (c) 2016, 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.
+
+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;
+/// this makes it possible for clients to use the front end in a way that
+/// doesn't require file system access (e.g. to run unit tests, or to run
+/// inside a browser).
+///
+/// Not intended to be implemented or extended by clients.
+abstract class FileSystem {
+  /// Returns a path context suitable for use with this [FileSystem].
+  path.Context get context;
+
+  /// Returns a [FileSystemEntity] corresponding to the given [path].
+  ///
+  /// Uses of `..` and `.` in path are normalized before returning (so, for
+  /// example, `entityForPath('./foo')` and `entityForPath('foo')` are
+  /// equivalent).  Relative paths are also converted to absolute paths.
+  ///
+  /// Does not check whether a file or folder exists at the given location.
+  FileSystemEntity entityForPath(String path);
+
+  /// Returns a [FileSystemEntity] corresponding to the given [uri].
+  ///
+  /// Uses of `..` and `.` in the URI are normalized before returning.  Relative
+  /// paths are also converted to absolute paths.
+  ///
+  /// If [uri] is not a `file:` URI, an [Error] will be thrown.
+  ///
+  /// Does not check whether a file or folder exists at the given location.
+  FileSystemEntity entityForUri(Uri uri);
+}
+
+/// Abstract representation of a file system entity that may or may not exist.
+///
+/// Not intended to be implemented or extended by clients.
+abstract class FileSystemEntity {
+  /// Returns the absolute normalized path represented by this file system
+  /// entity.
+  ///
+  /// Note: if the [FileSystemEntity] was created using
+  /// [FileSystem.entityForPath], this is not necessarily the same as the path
+  /// that was used to create the object, since the path might have been
+  /// normalized.
+  String get path;
+
+  /// Attempts to access this file system entity as a file and read its contents
+  /// as raw bytes.
+  ///
+  /// If an error occurs while attempting to read the file (e.g. because no such
+  /// file exists, or the entity is a directory), the future is completed with
+  /// an [Exception].
+  Future<List<int>> readAsBytes();
+
+  /// Attempts to access this file system entity as a file and read its contents
+  /// as a string.
+  ///
+  /// The file is assumed to be UTF-8 encoded.
+  ///
+  /// If an error occurs while attempting to read the file (e.g. because no such
+  /// file exists, or the entity is a directory), the future is completed with
+  /// an [Exception].
+  Future<String> readAsString();
+}