MdYamlDocLoader: Load the docs in another isolate

This will hopefully make opening the app a bit better as the yaml
parsing is now in another thread and frames would no longer be skipped.

lib/core/md_yaml_doc_loader.dart

@@ -1,23 +1,64 @@
 import 'dart:io';
+import 'dart:isolate';
 
 import 'package:gitjournal/core/md_yaml_doc.dart';
 import 'package:gitjournal/core/md_yaml_doc_codec.dart';
 
 class MdYamlDocLoader {
-  static final _serializer = MarkdownYAMLCodec();
+  Isolate _isolate;
+  ReceivePort _receivePort = ReceivePort();
+  SendPort _sendPort;
+
+  Future<void> _initIsolate() 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 {
-    // FIXME: What about parse errors?
+    await _initIsolate();
 
     final file = File(filePath);
     if (!file.existsSync()) {
       throw MdYamlDocNotFoundException(filePath);
     }
 
+    var rec = ReceivePort();
+    _sendPort.send(_LoadingMessage(filePath, rec.sendPort));
+
+    var data = await rec.first;
+    assert(data is MdYamlDoc);
+
+    return data;
+  }
+}
+
+class _LoadingMessage {
+  String filePath;
+  SendPort sendPort;
+
+  _LoadingMessage(this.filePath, this.sendPort);
+}
+
+void _isolateMain(SendPort toMainSender) {
+  ReceivePort fromMainRec = ReceivePort();
+  toMainSender.send(fromMainRec.sendPort);
+
+  final _serializer = MarkdownYAMLCodec();
+
+  fromMainRec.listen((data) async {
+    assert(data is _LoadingMessage);
+    var msg = data as _LoadingMessage;
+
+    final file = File(msg.filePath);
     final fileData = await file.readAsString();
     var doc = _serializer.decode(fileData);
-    return doc;
-  }
+
+    msg.sendPort.send(doc);
+  });
 }
 
 class MdYamlDocNotFoundException implements Exception {