Use URIs rather than paths in front end API.

pkg/front_end/lib/file_system.dart

 // 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.
   ///
-  /// If [uri] is not an absolute `file:` URI, an [Error] will be thrown.
+  /// If the URI scheme is not supported by this file system, 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.
 ///
 /// Instances of this class have suitable implementations of equality tests and
 /// hashCode.
 ///
 /// Not intended to be implemented or extended by clients.
 abstract class FileSystemEntity {
-  /// Returns the absolute normalized path represented by this file system
+  /// Returns the absolute normalized URI 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;
+  /// Note: this is not necessarily the same as the URI that was passed to
+  /// [FileSystem.entityForUri], since the URI might have been normalized.
+  Uri get uri;
 
   /// 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, the entity is a directory, or the file is not valid UTF-8),
   /// the future is completed with an [Exception].
   Future<String> readAsString();
 }