No. I'll put it you like this- the people who use drugs to put people to sleep- anesthesiologists- study for years to do this, they do it in a controlled environment in the operating theatre,and the drug mix and dose has to be personally tailored to the patient based on their medical history, physical exam, lab tests, etc, AND they constantly monitor the patient's vital signs throughout the operation. If there was a drug that could simply reliably put people to sleep safely with a single injection, do you honestly think they would be doing all this?
Sleep drugs are basically nothing like you see in the movies- there is no such thing as a special dart that will safely and reliably put anyone you hit with it to sleep. It has to be tailored to the individual person- for instance, if you got the dose required to disable a massive, drugged-up bodybuilder and gave it to a tiny young woman with a health problem, you would probably kill her several times over. Also, a dart will usually hit muscle rather than a vein, and it can take quite a while for drugs given there to do anything- potentially a few minutes at a minimum (anaesthesiologists give their drugs into a vein where the response is a few seconds. You can give sedative drugs into a muscle in the hospital for a violent patient but even that requires constant monitoring and observation).
Basically, there are no drugs or darts that would be effective for this. There are tasers but they need to be fairly close-range and they can fail- I once say the police shoot a taser into a drugged-out guy on PCP to basically zero effect (this may also have been due to the prongs landing too close together and the charge not spreading out enough to have any effect). Spray can work at close range as well.
In the Dallas situation, keep in mind that this was an ex-military guy sniping at the police from cover at range, so a lot of less-than-lethal options would not have been a realistic option for the police in that situation.