"When you use the Attack action and attack with a one handed weapon, you can use a bonus action to attack with a hand crossbow you are holding."
Originally Posted by 5e rules for Bonus Actions
You choose when to take a Bonus Action during Your Turn, unless the bonus action’s timing is specified.
While there is no "after" in Crossbow Expert, the wording does imply that you must have attacked with your 1-H weapon before you get the Bonus Action. A lenient ruling would allow you to declare that you're taking the Attack Action and your BA attack, so you could BA crossbow attack first but then you'd be required to take an Attack Action that turn.

Performing a ranged attack while adjacent to an enemy means that attack is made at Disadvantage. It does not provoke an AoO.

Unfortunately, the phrasing for two-weapon fighting is the same:
Originally Posted by 5e rules for Two-Weapon Fighting (a basic feature all classes get)
When you take the Attack action and Attack with a light melee weapon that you’re holding in one hand, you can use a Bonus Action to Attack with a different light melee weapon that you’re holding in the other hand. You don’t add your ability modifier to the damage of the bonus Attack, unless that modifier is negative.

If either weapon has the Thrown property, you can throw the weapon, instead of making a melee Attack with it.
Again, we're in this weird space where RAW isn't clear. The timing is not specified, but the Attack action is a prerequisite of the BA attack...
Imo most reasonable DMs would rule that yes, you can throw a dart first. But I'm not 100% sure that this is RAW.

5e rules are written so poorly...Even Lead Rules Designer Jeremy Crawford has given conflicting answers on these questions.