Add integration test: hot reload + incremental compiler

pkg/front_end/lib/src/testing/hybrid_file_system.dart

+// Copyright (c) 2017, 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.
+
+/// A memory + physical file system used to mock input for tests but provide
+/// sdk sources from disk.
+library front_end.src.hybrid_file_system;
+
+import 'dart:async';
+
+import 'package:front_end/file_system.dart';
+import 'package:front_end/memory_file_system.dart';
+import 'package:front_end/physical_file_system.dart';
+
+/// A file system that mixes files from memory and a physical file system. All
+/// memory entities take priotity over file system entities.
+class HybridFileSystem implements FileSystem {
+  final MemoryFileSystem memory;
+  final PhysicalFileSystem physical = PhysicalFileSystem.instance;
+
+  HybridFileSystem(this.memory);
+
+  @override
+  FileSystemEntity entityForUri(Uri uri) => new HybridFileSystemEntity(
+      memory.entityForUri(uri), physical.entityForUri(uri));
+}
+
+/// Entity that delegates to an underlying memory or phisical file system
+/// entity.
+class HybridFileSystemEntity implements FileSystemEntity {
+  final FileSystemEntity memory;
+  final FileSystemEntity physical;
+
+  HybridFileSystemEntity(this.memory, this.physical);
+
+  FileSystemEntity _delegate;
+  Future<FileSystemEntity> get delegate async {
+    return _delegate ??= (await memory.exists()) ? memory : physical;
+  }
+
+  @override
+  Uri get uri => memory.uri;
+
+  @override
+  Future<bool> exists() async => (await delegate).exists();
+
+  @override
+  Future<DateTime> lastModified() async => (await delegate).lastModified();
+
+  @override
+  Future<List<int>> readAsBytes() async => (await delegate).readAsBytes();
+
+  @override
+  Future<String> readAsString() async => (await delegate).readAsString();
+}