Add integration test: hot reload + incremental compiler

pkg/front_end/lib/src/vm/reload.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 helper library to connect to an existing VM and trigger a hot-reload via
+/// its service protocol.
+///
+/// Usage:
+///
+/// ```
+///     var reloader = new VmReloader();
+///     await reloader.reload(uriToEntryScript);
+///     ...
+///     await reloader.disconnect();
+/// ```
+library front_end.src.vm.reload;
+
+import 'dart:async';
+import 'package:json_rpc_2/json_rpc_2.dart' as json_rpc;
+import 'package:stream_channel/stream_channel.dart';
+import 'package:web_socket_channel/io.dart';
+
+/// A user API to trigger hot reloads on a running VM via the VM's service
+/// protocol.
+class VmReloader {
+  /// Port used to connect to the vm service protocol, typically 8181.
+  final int port;
+
+  /// An peer point used to send service protocol messages. The service
+  /// protocol uses JSON rpc on top of web-sockets.
+  json_rpc.Peer get rpc => _rpc ??= _createPeer();
+  json_rpc.Peer _rpc;
+
+  /// The main isolate ID of the running VM. Needed to indicate to the VM which
+  /// isolate to reload.
+  FutureOr<String> get mainId async => _mainId ??= await _computeMainId();
+  String _mainId;
+
+  VmReloader([int port = 8181]) : port = port;

[scheglov, 2017/06/07 00:27:23]

[this.port = 8181] and you don't need "port = port" then.

+
+  /// Establishes the JSON rpc connection.
+  _createPeer() {

[scheglov, 2017/06/07 00:27:23]

Please add the return type.

+    StreamChannel socket =
+        new IOWebSocketChannel.connect('ws://127.0.0.1:$port/ws');
+    var peer = new json_rpc.Peer(socket);
+    peer.listen().then((_) {
+      if (VERBOSE_DEBUG) print('connection to vm-service closed');
+      return disconnect();
+    }).catchError((e) {
+      if (VERBOSE_DEBUG) print('error connecting to the vm-service');
+      return disconnect();
+    });
+    return peer;
+  }
+
+  /// Retrieves the ID of the main isolate using the service protocol.
+  Future<String> _computeMainId() async {
+    var vm = await rpc.sendRequest('getVM');
+    var isolates = vm['isolates'];
+    for (var isolate in isolates) {
+      if (isolate['name'].contains('\$main')) {

[scheglov, 2017/06/07 00:27:23]

I that think using r'$main' would show that we ourself use this nice feature of
Dart :-)

+        return isolate['id'];
+      }
+    }
+    return isolates.first['id'];
+  }
+
+  /// Send a request to the VM to reload sources from [entryUri].
+  ///
+  /// This will establish a connection with the VM assuming it is running on the
+  /// local machine and listening on [port] for service protocol requests.

[scheglov, 2017/06/07 00:27:23]

Please document and type what is returned.

+  Future reload(Uri entryUri) async {
+    var id = await mainId;
+    var result = await rpc.sendRequest('reloadSources', {
+      'isolateId': id,
+      'rootLibUri': entryUri.path,
+    });
+    return result;
+  }
+
+  /// Close any connections used to communicate with the VM.
+  disconnect() async {

[scheglov, 2017/06/07 00:27:23]

Please add the return type.

+    if (_rpc == null) return null;
+    this._mainId = null;
+    if (!_rpc.isClosed) {
+      var future = _rpc.close();
+      _rpc = null;
+      return future;
+    }
+    return null;
+  }
+}
+
+const VERBOSE_DEBUG = false;