DateTime constructor should have an optional, named utc parameter

[jamesderlin]

It's quite annoying that DateTime has separate constructors for non-UTC and UTC DateTimes instead of offering a single constructor with a boolean flag. This makes attempts to implement copyWith-like methods (see https://github.com/dart-lang/sdk/issues/24644) rather tedious and error-prone since they must invoke different constructors based on isUtc and duplicate all of the arguments.

Of course, this wouldn't be a problem if:

• There were constructor tear-offs, and then implementations could do (dateTime.isUtc ? DateTime.utc, DateTime.new)(...). (Are those coming soon? If so, then this issue is probably moot.)
• DateTime provided a copyWith method (which is https://github.com/dart-lang/sdk/issues/24644).

[lrhn]

That's not currently possible. The DateTime constructor has optional positional parameters, so it cannot also have a named parameter.

Now, if we allowed both (https://github.com/dart-lang/language/issues/1076), that would probably have been the design all along. If we add that ability, I promise I'll personally put a {bool utc = false} parameter on the DateTime constructor. (My current desired approach is to remove the notion of declaring optional parameters entirely, and make any nullable parameter optional.)

There will be constructor tear-offs.

[spydon]

There are now constructor tear-offs, so that can be used as a work around, meanwhile we eagerly wait for https://github.com/dart-lang/language/issues/1076