In a small bowl or liquid measuring cup, whisk together the broth, soy sauce, 2 teaspoons cornstarch, vinegar, red pepper flakes and sugar until smooth. Set aside.
In a medium bowl, toss the chicken with 1 teaspoon salt, 1/2 teaspoon pepper and remaining 3 teaspoons cornstarch.
In a large 12-inch nonstick skillet, heat the 2 teaspoons oil over medium heat until hot and rippling. Add the chicken, breaking up any clumps, and cook until it is golden brown on all sides and just barely cooked through (it will cook a few minutes longer later in the recipe). Scrape the chicken onto a plate, keeping as much liquid in the skillet as possible.
Add the garlic, bell pepper, and onion and cook for 3-4 minutes, until the vegetables are crisp-tender.
Return the chicken and any accumulated juices to the skillet over medium heat. Whisk the sauce quickly before pouring it into the skillet. Add the mango (or pineapple). Cook, stirring constantly, while the sauce simmers and thickens, 1-2 minutes. Season with additional salt and pepper to taste, if needed.