Use URIs rather than paths in front end API.

pkg/front_end/lib/physical_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.physical_file_system;
 
 import 'dart:async';
 import 'dart:io' as io;
 
 import 'package:path/path.dart' as p;
 
 import 'file_system.dart';
 
 /// Concrete implementation of [FileSystem] which performs its operations using
 /// I/O.
 ///
 /// Not intended to be implemented or extended by clients.
 class PhysicalFileSystem implements FileSystem {
   static final PhysicalFileSystem instance = new PhysicalFileSystem._();
 
   PhysicalFileSystem._();
 
   @override
   p.Context get context => p.context;
 
   @override
-  FileSystemEntity entityForPath(String path) =>
-      new _PhysicalFileSystemEntity(context.normalize(context.absolute(path)));
-
-  @override
   FileSystemEntity entityForUri(Uri uri) {
-    if (uri.scheme != 'file') throw new ArgumentError('File URI expected');
+    if (uri.scheme != 'file' && uri.scheme != '') {
+      throw new ArgumentError('File URI expected');
+    }
     // Note: we don't have to verify that the URI's path is absolute, because
     // URIs with non-empty schemes always have absolute paths.
-    return entityForPath(context.fromUri(uri));
+    var path = context.fromUri(uri);
+    return new _PhysicalFileSystemEntity(
+        context.normalize(context.absolute(path)));
   }
 }
 
 /// Concrete implementation of [FileSystemEntity] for use by
 /// [PhysicalFileSystem].
 class _PhysicalFileSystemEntity implements FileSystemEntity {
-  @override
-  final String path;
+  final String _path;
 
-  _PhysicalFileSystemEntity(this.path);
+  _PhysicalFileSystemEntity(this._path);
 
   @override
-  int get hashCode => path.hashCode;
+  int get hashCode => _path.hashCode;
+
+  @override
+  Uri get uri => p.toUri(_path);
 
   @override
   bool operator ==(Object other) =>
-      other is _PhysicalFileSystemEntity && other.path == path;
+      other is _PhysicalFileSystemEntity && other._path == _path;
 
   @override
-  Future<List<int>> readAsBytes() => new io.File(path).readAsBytes();
+  Future<List<int>> readAsBytes() => new io.File(_path).readAsBytes();
 
   @override
-  Future<String> readAsString() => new io.File(path).readAsString();
+  Future<String> readAsString() => new io.File(_path).readAsString();
 }