Colorful Chicken Stew

1 lb. boneless skinless chicken breasts, cubed
1 (14 1/2 oz.) can Italian diced tomatoes, undrained
2 medium potatoes, peeled and cut into 1/2−inch cubes
5 medium carrots, chopped
3 celery ribs, chopped
1 large onion, chopped
1 medium green bell pepper, chopped
2 (4 oz.) cans mushroom stems and pieces, drained
2 low−sodium chicken bouillon cubes
Artificial Sweetener equal to 2 tsp. sugar
1 tsp. chili powder
1/4 teaspoon pepper
1 tablespoon cornstarch
2 cup cold water