Use URIs rather than paths in front end API.

pkg/front_end/lib/memory_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.memory_file_system;
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:typed_data';
 
 import 'package:path/path.dart' as p;
 
 import 'file_system.dart';
 
 /// Concrete implementation of [FileSystem] which performs its operations on an
 /// in-memory virtual file system.
 ///
 /// Not intended to be implemented or extended by clients.
 class MemoryFileSystem implements FileSystem {
   @override
   final p.Context context;
 
-  final Map<String, Uint8List> _files = {};
+  final Map<Uri, Uint8List> _files = {};
 
   /// The "current directory" in the in-memory virtual file system.
   ///
   /// This is used to convert relative paths to absolute paths.
-  String currentDirectory;
+  ///
+  /// Always ends in a trailing '/'.
+  Uri currentDirectory;
 
-  MemoryFileSystem(this.context, this.currentDirectory);
-
-  @override
-  MemoryFileSystemEntity entityForPath(String path) =>
-      new MemoryFileSystemEntity._(
-          this, context.normalize(context.join(currentDirectory, path)));
+  MemoryFileSystem(this.context, Uri currentDirectory)
+      : currentDirectory = _addTrailingSlash(currentDirectory);
 
   @override
   MemoryFileSystemEntity entityForUri(Uri uri) {
-    if (uri.scheme != 'file') 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));
+    return new MemoryFileSystemEntity._(
+      this, currentDirectory.resolveUri(uri).normalizePath());
+  }
+
+  static Uri _addTrailingSlash(Uri uri) {
+    if (!uri.path.endsWith('/')) {
+      uri = uri.replace(path: uri.path + '/');
+    }
+    return uri;
   }
 }
 
 /// Concrete implementation of [FileSystemEntity] for use by
 /// [MemoryFileSystem].
 class MemoryFileSystemEntity implements FileSystemEntity {
   final MemoryFileSystem _fileSystem;
 
   @override
-  final String path;
+  final Uri uri;
 
-  MemoryFileSystemEntity._(this._fileSystem, this.path);
+  MemoryFileSystemEntity._(this._fileSystem, this.uri);
 
   @override
-  int get hashCode => path.hashCode;
+  int get hashCode => uri.hashCode;
 
   @override
   bool operator ==(Object other) =>
       other is MemoryFileSystemEntity &&
-      other.path == path &&
+      other.uri == uri &&
       identical(other._fileSystem, _fileSystem);
 
   @override
   Future<List<int>> readAsBytes() async {
-    List<int> contents = _fileSystem._files[path];
+    List<int> contents = _fileSystem._files[uri];
     if (contents != null) {
       return contents.toList();
     }
     throw new Exception('File does not exist');
   }
 
   @override
   Future<String> readAsString() async {
     List<int> contents = await readAsBytes();
     return UTF8.decode(contents);
   }
 
   /// Writes the given raw bytes to this file system entity.
   ///
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsBytesSync(List<int> bytes) {
-    _fileSystem._files[path] = new Uint8List.fromList(bytes);
+    _fileSystem._files[uri] = new Uint8List.fromList(bytes);
   }
 
   /// Writes the given string to this file system entity.
   ///
   /// The string is encoded as UTF-8.
   ///
   /// If no file exists, one is created.  If a file exists already, it is
   /// overwritten.
   void writeAsStringSync(String s) {
     // Note: the return type of UTF8.encode is List<int>, but in practice it
     // always returns Uint8List.  We rely on that for efficiency, so that we
     // don't have to make an extra copy.
-    _fileSystem._files[path] = UTF8.encode(s) as Uint8List;
+    _fileSystem._files[uri] = UTF8.encode(s) as Uint8List;
   }
 }