compose(style1: ?StyleProp, style2: ?StyleProp): ?StyleProp {
    if (style1 && style2) {
      return [style1, style2];
    } else {
      return style1 || style2;
    }
  }
Combines two styles such that `style2` will override any styles in `style1`.
If either style is falsy, the other one is returned without allocating an
array, saving allocations and maintaining reference equality for
PureComponent checks.