[augmentations] No error when augmenting and introductory constructor both have redirection

[sgrekhov]

Feature specification:

Redirecting generative constructors
...
It is a compile-time error if:

• The augmented constructor has an initializer list or a body, or it has a redirection.

But the code below has no such error in the analyzer.

class C {
  int x;
  C(this.x);
  C.foo(int x);
}

augment class C {
  augment C.foo(int x) : this(x + 1); // Ok
  augment C.foo(int x) : this(x + 2); // Augmented constructor already has a redirection
//                       ^
// [analyzer] unspecified
// [cfe] unspecified
}

Appropriate co19 test