This came to light because of this issue report:
https://github.com/google/grumpy/issues/305

Basically, map() and zip() were detecting StopIteration but were calling
ExcRestore(nil, nil) in the wrong place, so under normal circumstances,
exc info would be set to StopIteration after they returned.

Normally this would not cause a problem, but the code in the bug report
was calling a native function (time.Now()) subsequent to a map() and
nativeInvoke checks ExcInfo() to determine whether the Go function
manually triggered an exception. See:

https://github.com/google/grumpy/blob/master/runtime/native.go#L572

In this case, nativeInvoke was being fooled because exc info had been
set previously. nativeInvoke should probably be smarter about this by
resetting the exception before the native call.