Heat vegetable oil in a 12-inch nonstick skillet over moderately high heat until hot but not smoking, then stir-fry garlic, ginger, and red pepper flakes until fragrant, about 1 minute.
Add sugar snaps and snow peas and stir-fry until crisp-tender, about 3 minutes.
Add frozen peas and stir-fry until hot, about 2 minutes.
Remove from heat, then stir in soy sauce and sesame oil.