Allow optional arguments in synthetic constructors for mixins

[karlklose]

The rules for synthetic constructors from superclasses in mixins do not cover optional arguments, implying that constructors with optional arguments are not available to call as super-constructors from the class that extends the mixin application:

Section 9.1:
For each generative constructor named
  q_i(T_i1 a_i1; ... ; T_iki a_iki)
C has an implicitly declared constructor named
  q_0i(a_i1; ... ; aiki) : super(a_i1; ... ; a_iki);

Is this on purpose?

It means that the following example does not work:
  class A {
    A([this.o]);
  }

  class M {}

  class B extends A with M {
    B([o]) : super(o);
  }

  main() => new B(1);

and it actually fails in the VM with the error message:
  forwarding constructors must not have optional parameters
  class B extends A with M {
                         ^

[gbracha]

Really all the restrictions on mixins need to go away. They are all driven by short term implementation considerations. I doubt if we will change this before the first ECMA standard is approved, but we'll see.

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Set owner to @gbracha.
Added Accepted label.

[crelier]

See also issues 1501 and 19226.
Improved error message suggesting workaround submitted as r37057.

[gbracha]

Added Duplicate label.
Marked as being merged into #15101.