in packages/runtime/runtime.js [124:132]
exports.isGeneratorFunction = function(genFun) {
var ctor = typeof genFun === "function" && genFun.constructor;
return ctor
? ctor === GeneratorFunction ||
// For the native GeneratorFunction constructor, the best we can
// do is to check its .name property.
(ctor.displayName || ctor.name) === "GeneratorFunction"
: false;
};