Required parameters in constructors vs. property bag in from()

[ptomato]

In new Temporal.DateTime(y, mon, d, h, min, s, ms, µs, ns) the first five parameters are required. Whereas, Temporal.DateTime.from(propertyBag) is quite different: In the spec, all the parameters are required (see the ToDateTime operation.) In the polyfill, none are required although at least one must be present (from({}) doesn't work.)

All of this seems inconsistent to me. Taking account of the discussion in #41, why would half the constructor parameters be optional and half required, when there are (IMO) reasonable default values for all of them? And I'd argue that if you can't call new Temporal.DateTime(2000) because there's no reasonable default value, then you shouldn't be able to call Temporal.DateTime.from({year: 2000}), and conversely if you can do the second one then you should be able to do the first one.

I'd propose making all the parameters optional everywhere. If you can call Temporal.DateTime.from({month: 5}) and get midnight, May 1, 1BCE then why not {} for midnight, January 1, 1BCE? And it would allow doing stuff like Temporal.DateTime.from(monthDayObject).with({year: 2000}) to get the the 2000 occurrence of the calendar month from your Temporal.MonthDay object.

[sffc]

From 2020-02-19 Temporal call: year, month, and day should be required; all others should be optional. Add calendar as the last argument.

[sffc]

One question: are year, month, and day required in both the constructor and .from(), or only in the constructor? I'd say we should require them to all be present in both places. If you want a year/month or a month/day, we have types for that.

[ptomato]

In the PR attached here, they are all required in both places.