I’ve now seen Christopher Wheeldon’s pas de deux from “After the Rain” three times. It is a quiet piece, all pale leotards and glowing late-afternoon light; the dancers’ steps are small and sometimes tentative, sometimes languorous, yet the entire work brims with the power of sweet sadness, or perhaps sad sweetness—some fleeting emotion distilled to the precise tension in the dancers’ bodies, the peculiar movements of their heads and arms and feet.