Error stack strawman

[Gary Guo]

Things became more complicated when considering async & generators.

For async calls as mentioned before somewhere in the thread, only Firefox Nightly build includes histories of the frame, while all other browsers don't. It could be useful to include that in stack trace, but it will then be a special case to a general rule, so not all functions are treated as the same. Also, including this information can be costly.

Similar reasoning about usefulness can be extended to generators & async functions. Shall we include a copy of history frames at the time the function is called? (If we use FF's SavedFrame format, then in this case both `parent` and `asyncParent` will be set) Again, this will be costly as well.

In these two cases, it could be useful to developers if these information are available, but generally these will be costly to collect. Making these optional can be a choice, by simply not including unsupported fields in the returned object. I suppose `isTailCall` can be made optional as well. 		 	   		  
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