Detect if ses is being transformed

[kumavis]

it would be helpful for debugging if there were some heuristics for checking if the ses dists were being transformed, as in #659

even being woefully imperfect, this check can be useful by logging a warning or error suggesting that a transform might have taken place.

one common thing that is easy to check is if async functions are being transformed to non-async functions.

here is an example of the transformation.

original:

const testResult = Object.getPrototypeOf(async function AsyncFunctionInstance() {}) === Function.prototype
//=> normally false, but true after transformation

after transformation:

var testResult = Object.getPrototypeOf(function AsyncFunctionInstance() {
  return regeneratorRuntime.async(function AsyncFunctionInstance$(_context) {
    while (1) {
      switch (_context.prev = _context.next) {
        case 0:
        case "end":
          return _context.stop();
      }
    }
  }, null, null, null, Promise);
}) === Function.prototype;
//=> true

note that the transformed async function has a prototype of Function.prototype. we could throw an error suggesting that the code might have been transformed.

[dckc]

...

one common thing that is easy to check is if async functions are being transformed to non-async functions.

Any chance you can go back about a month or two and add that before I/we spent umpteen hours debugging that situation?

;-)

That is: yes, let's please do. Good idea.

[erights]

Added the security label because such unexpected translation is also a security hazard, and detecting it sometimes would reduce this hazard.