Firefox and WebKit require native promises to provide awaitPromise
functionality. When the Promise is overwritten, all evaluations
in the main world produce wrong Promise, so we wrap with async
function to get a native promise instead.
We currently return undefined whenever we had an error trying
return the evaluation result by error. The most common error
is "execution context destroyed".
This produces very unexpected undefined from methods that do not
ever expect undefined. Instead, we should throw because we were
not able to return the result.
