Fix tests

lib/src/models/documents/history.dart

@@ -1,5 +1,3 @@
-import 'package:meta/meta.dart' show immutable;
-
 import '../quill_delta.dart';
 import '../structs/doc_change.dart';
 import '../structs/history_changed.dart';
@@ -14,7 +12,7 @@ class History {
     this.lastRecorded = 0,
   });
 
-  final HistoryStack stack = const HistoryStack.empty();
+  HistoryStack stack = HistoryStack.empty();
 
   bool get hasUndo => stack.undo.isNotEmpty;
 
@@ -121,14 +119,13 @@ class History {
   }
 }
 
-@immutable
 class HistoryStack {
-  const HistoryStack.empty()
-      : undo = const [],
-        redo = const [];
+  HistoryStack.empty()
+      : undo = [],
+        redo = [];
 
-  final List<Delta> undo;
-  final List<Delta> redo;
+  List<Delta> undo;
+  List<Delta> redo;
 
   void clear() {
     undo.clear();