The Event and CustomEvent constructors diverged from Chrome in three
observable ways:
- new Event() / new CustomEvent() with no argument did not throw, though the
type argument is required (Chrome: "1 argument required, but only 0
present."). initEvent/initMouseEvent already had this check; the
constructors did not.
- the type argument was not coerced to a string, so new Event(123).type was
the number 123 instead of "123".
- CustomEvent.detail defaulted to undefined; WebIDL CustomEventInit specifies
`any detail = null`.
Add the required-argument guard to both constructors, coerce type via
String(t) (inherited by every subclass through super), and default detail to
null. createEvent() builds events with an explicit '' type, so the arity
guard leaves the type-"" uninitialized-event path intact.
Fixes#552
Andrey Mnatsakanov
af1e15f6c8