Use provided methods instead of protected members

qutebrowser/browser/browsertab.py

@@ -806,7 +806,7 @@ class AbstractTab(QWidget):
         raise NotImplementedError
 
     def dump_async(self, callback, *, plain=False):
-        """Dump the current page to a file ascync.
+        """Dump the current page's html asynchronously.
 
         The given callback will be called with the result when dumping is
         complete.

qutebrowser/browser/commands.py

@@ -1515,10 +1515,10 @@ class CommandDispatcher:
             return
         if current_url.scheme() == 'view-source':
             raise cmdexc.CommandError("Already viewing source!")
+
         if edit:
             ed = editor.ExternalEditor(self._tabbed_browser)
-
-            tab._widget.page().toHtml(ed.edit)
+            tab.dump_async(ed.edit)
         else:
             tab.action.show_source()