Add implementations of the front end FileSystem 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 = {};
+
+  /// The "current directory" in the in-memory virtual file system.
+  ///
+  /// This is used to convert relative paths to absolute paths.
+  String currentDirectory;
+
+  MemoryFileSystem(this.context, this.currentDirectory);
+
+  @override
+  MemoryFileSystemEntity entityForPath(String path) =>
+      new MemoryFileSystemEntity._(
+          this, context.normalize(context.join(currentDirectory, path)));
+
+  @override
+  MemoryFileSystemEntity entityForUri(Uri uri) {
+    if (uri.scheme != 'file') throw new ArgumentError('File URI expected');

[scheglov, 2016/11/03 02:15:24]

Do we want to check (or TODO) that uri is absolute?

[Paul Berry, 2016/11/03 15:16:41]

AFAICT, the scheme check is sufficient--it's impossible to create a relative URI
with a non-empty scheme.  Even if you do `new Uri(scheme: 'file', path:
'file.txt')`, the path winds up being changed to '/file.txt'.

I've added a comments to clarify (both here and in PhysicalFileSystem), and I've
added tests just to be certain.

+    return entityForPath(context.fromUri(uri));
+  }
+}
+
+/// Concrete implementation of [FileSystemEntity] for use by
+/// [MemoryFileSystem].
+class MemoryFileSystemEntity implements FileSystemEntity {
+  final MemoryFileSystem _fileSystem;
+
+  @override
+  final String path;
+
+  MemoryFileSystemEntity._(this._fileSystem, this.path);
+
+  @override
+  int get hashCode => path.hashCode;
+
+  @override
+  bool operator ==(Object other) =>
+      other is MemoryFileSystemEntity && other.path == path;

[Siggi Cherem (dart-lang), 2016/11/03 00:46:50]

I'm ok not adding it, but any need to also check that the _fileSystem is the
same?

[Paul Berry, 2016/11/03 15:16:41]

Done.

+
+  @override
+  Future<List<int>> readAsBytes() async {
+    List<int> contents = _fileSystem._files[path];
+    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);
+  }
+
+  /// 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) {
+    _fileSystem._files[path] = UTF8.encode(s);

[scheglov, 2016/11/03 02:15:23]

AFAIK UTF8 returns Uint8List at runtime, but the static type is List<int>.

[Paul Berry, 2016/11/03 15:16:41]

True.  That means that technically this implementation relies on an
implementation detail of UTF8.encode.  I think that's ok--it's an implementation
detail that seems really unlikely to be broken in the future, and if it ever is
broken the test will fail.

I've added a comment and a cast (`as Uint8List`) to make this more explicit.  If

+  }
+}