Note: Never let the created be null

Null paramters are annoying, cause we then need to check if != null all
the time. I hate this about Dart. A type should always have sensible
defaults.

lib/note.dart

@@ -12,6 +12,10 @@ class Note implements Comparable {
   Map<String, dynamic> extraProperties = new Map<String, dynamic>();
 
   Note({this.created, this.body, this.fileName, this.extraProperties}) {
+    assert(this.created != null);
+    assert(this.body != null);
+    assert(this.fileName != null);
+
     if (extraProperties == null) {
       extraProperties = new Map<String, dynamic>();
     }

lib/storage/serializers.dart

@@ -28,7 +28,7 @@ class MarkdownYAMLSerializer implements NoteSerializer {
       var parts = str.split("---\n");
 
       var yamlMap = loadYaml(parts[1]);
-      Map<String, dynamic> map = new Map<String, dynamic>();
+      var map = new Map<String, dynamic>();
       yamlMap.forEach((key, value) {
         map[key] = value;
       });
@@ -37,7 +37,9 @@ class MarkdownYAMLSerializer implements NoteSerializer {
       return new Note.fromJson(map);
     }
 
-    return new Note(body: str);
+    var map = new Map<String, dynamic>();
+    map['body'] = str;
+    return new Note.fromJson(map);
   }
 
   @override