MdYamlDocLoader: Avoid starting the isolate multiple times

Man, there is a lot of locking that I hadn't even thought of while
writing all of this code.

lib/core/md_yaml_doc_loader.dart

@@ -1,6 +1,8 @@
 import 'dart:io';
 import 'dart:isolate';
 
+import 'package:synchronized/synchronized.dart';
+
 import 'package:gitjournal/core/md_yaml_doc.dart';
 import 'package:gitjournal/core/md_yaml_doc_codec.dart';
 
@@ -9,13 +11,19 @@ class MdYamlDocLoader {
   ReceivePort _receivePort = ReceivePort();
   SendPort _sendPort;
 
+  var _loadingLock = Lock();
+
   Future<void> _initIsolate() async {
     if (_isolate != null) return;
+
+    return await _loadingLock.synchronized(() async {
+      if (_isolate != null) return;
       _isolate = await Isolate.spawn(_isolateMain, _receivePort.sendPort);
 
       var data = await _receivePort.first;
       assert(data is SendPort);
       _sendPort = data as SendPort;
+    });
   }
 
   Future<MdYamlDoc> loadDoc(String filePath) async {

pubspec.lock

@@ -640,6 +640,13 @@ packages:
       url: "https://pub.dartlang.org"
     source: hosted
     version: "1.0.5"
+  synchronized:
+    dependency: "direct main"
+    description:
+      name: synchronized
+      url: "https://pub.dartlang.org"
+    source: hosted
+    version: "2.2.0"
   term_glyph:
     dependency: transitive
     description:

pubspec.yaml

@@ -36,6 +36,7 @@ dependencies:
   connectivity: ^0.4.6+1
   flutter_emoji: ">= 2.0.0"
   git_url_parse2: ^0.0.1
+  synchronized: ^2.2.0
 
 dev_dependencies:
   flutter_launcher_icons: "^0.7.2"