Make mixins forward const constructors.

[lrhn]

Currently mixin application is specified as creating forwarding constructors for all generative constructors of the superclass.
The created constructors are not const constructors, making it impossible to create a const class with a mixin.

If the superclass constructor is a const constructor, and the mixin doesn't add any fields, please make the created constructor also a const constructor. That would make it possible to create a const class using a mixin:

  class EmptyList<E> extends Object with ImmutableListMixin<E> {
    int get length => 0;
    int operator[](int i) { throw new RangeError.value(i); }
  }

  ... return const EmptyList(); ...

Not having const constructors makes mixins less useful.

[gbracha]

The restrictions on mixins should be relaxed, but not by adding more special rules. We've had this discussion and concluded that the spec will remain as it is until full constructor support is allowed in mixins.

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Set owner to @gbracha.
Added this to the Later milestone.
Added Accepted label.

[lrhn]

Issue #18707 has been merged into this issue.

[kasperl]

Removed this from the Later milestone.
Added Oldschool-Milestone-Later label.

[kasperl]

Removed Oldschool-Milestone-Later label.

[eernstg]

Issue #29396 was closed as a duplicate of this one; check out that issue for the example.

[eernstg]

Closing: As of 63c6851, constructor forwarding does just that. Created #32223 to put this into the implementation pipeline.