CHICAGO — Three stories above a busy stretch of North Halsted Street, on a rooftop with views of downtown skyscrapers, crickets chirp and prairie flowers dance in the breeze. Waist-high grasses set the tone in a sprawling 2,500-square-foot native garden dotted with bright blooms: golden brown-eyed Susan, purple hoary vervain, yellow stiff goldenrod, snowy white turtlehead, lime-tinged spotted ...