Cut the roast beef into 1" chunks, trimming off any large pieces of fat.
In a large bowl, combine the green chilis, brown sugar, minced garlic, oil, lime juice, cumin, salt, pepper and red pepper flakes. Add the beef and toss, then marinate for at least 2 hours, or up to 24 hours.
Add the beef and the marinade to the slow cooker, cover, and cook on high for 3 hours or on low for 4 ½ hours—until you can easily shred the meat with 2 forks.
Preheat the oven to broil, then remove the beef from the slow cooker. Shred it using 2 forks or your hands (if it doesn't shred easily, return to the slow cooker and continue to cook). Transfer to a baking dish and pour about 1 cup of the drippings over the beef.
Broil for about 10 minutes, stirring the meat every 2 minutes, or until the meat is lightly golden.
Serve immediately with corn or flour tortillas, shredded cheese, sour cream, hot sauce and any other taco toppings you love. Enjoy!