You have the ability to make bulk changes to tasks in Dart via the list layout.

To start, make sure you've chosen the list layout, then, make sure the properties you would like to bulk change are visible as a column on your screen. If you need to make the visible, go to the "more" icon ("...") visible to the right of your existing columns:

In list view, to the left of the task title is a hidden check box. You'll be able to reveal and click on it by hovering over the task title itself:

To make bulk changes to multiple tasks, select the tasks you want to make the changes to. Then, hover over the section of the column to reveal the property icon within one of the highlighted tasks to make the change you desire. Every task selected will get the same change applied: