runtime.js should read Symbol values dynamically

[matthieusieben]

runtime.js currently caches the values of Symbol.iterator, Symbol.asyncIterator and Symbol.toStringTag when it is loaded (here).

This can cause issues when regenerator-runtime is loaded before the Symbol polyfills. Here is an example of Babel build that fails running on IE11 due to the order of the imports:

I did create an issue in Babel's repository. However, since regenerator is used by more than just Babel, and that this behavior can also affect other tools, I believe it would be better for regenerator-runtime to read the Symbol values wen invoked rather than when loaded. What do you think ?

[nyngwang]

I hope someone can fix this soon.