Allow new calls to classes with factories

[floitschG]

Currently it is not allowed (static warning) to 'new' a class that is abstract. This holds, even if the invoked constructor is a factory that in turn invokes non-abstract classes.
It would be convenient if this restriction would be lifted.
In the following example the Boolean class is abstract, but the factory instantiates non-abstract classes.
===
class Boolean {
  factory Boolean(value) {
    return value ? new True() : new False();
  }
  abstract foo();
}

class True extends Boolean {
  True();
  foo() => ...;
}
===

[gbracha]

This sounds reasonable, though we need to think carefully.

One of the advantages of factory constructors is that the user of the constructor can be oblivious to whether the constructor is a generative constructor or a factory. Does having a special allowance jeopardize that invariant?

It seems not, because if the factory was to replaced with a generative constructor, the class would cease to be abstract.

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Added Accepted label.

[DartBot]

This comment was originally written by @seaneagan

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similarly, how about allowing default implementations of abstract classes as is allowed with interfaces:

abstract class Abstract default NonAbstract { ...

[gbracha]

Sean, please file that as a separate issue.

[gbracha]

The 0.09 draft addresses this problem in sections 10.10.1 and 10.10.2.

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Added Done label.