[analyzer] Report a compile-time error for external initializing formals

[eernstg]

Consider the following program:

class A {
  final int i;
  external A(this.i);
}

void main() {}

The CFE reports a compile-time error on this library. The analyzer accepts it with no errors.

It was previously only mentioned in commentary that it is a compile-time error for an external generative constructor to have an initializing formal parameter, but in dart-lang/language#3517 the specification was clarified such that it is now clearly specified to be a compile-time error.

This issue is the implementation issue for that implementation effort. It is 'area-analyzer' rather than 'area-meta', with sub-issues, because there is nothing to implement in any other tool than the analyzer.

It is not a breaking change, because every external constructor declaration that has an initializing formal is reported as a compile-time error by the CFE already today.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/344180

[sgrekhov]

Seems that the issue still exists for optional parameters. The following tests fail (no expected errors):

co19/Language/Classes/Constructors/Generative_Constructors/initializing_formal_parameter_A02_t02
co19/Language/Classes/Constructors/Generative_Constructors/initializing_formal_parameter_A02_t03
co19/Language/Classes/Constructors/Generative_Constructors/initializing_formal_parameter_A02_t04

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/345081