[Wildcard Variables] co19 Tests

[kallentu]

Write up tests for co19. "Possible Test Cases" mirrors the test list for dart-lang/sdk#55652.
Examples pulled from the spec.

Possible Test Cases

May not be exhaustive.

• Multiple _ are allowed in function parameters. This includes top-level functions, local functions, function expressions ("lambdas"), instance methods, static methods, constructors, etc. It includes all parameter kinds: simple, field formals, and function-typed formals, etc.:

Foo(_, this._, super._, void _(), {_}) {}

list.where((_) => true);

• Multiple _ are allowed in local variable declaration statement variables.
• Multiple _ are allowed in for loop variable declarations.
• Multiple _ are allowed in catch clause parameters.
• Multiple _ are allowed in generic type and generic function type parameters.
• Other declarations can still be named _ as they are today: top-level variables, top-level function names, type names, member names, etc. are unchanged.
• _ is still a wildcard in patterns

int a;
(_, a) = (1, 2); // Valid.

• Error if we cannot resolve _ to a member or top-level declaration.

main() {
  _ = 1; // Error.
}

class C {
  var _;

  test() {
    _ = 2; // OK.
  }
}

• Wildcards do not shadow.

class C {
  var _ = 'field';

  test() {
    var _ = 'local';

    _ = 'assign'; // Assigns to the field and not the local.
  }
}

• A positional initializing formal named _ does still initialize a field named _ (and you can still have a field with that name)

class C {
  var _;

  C(this._); // OK.
}

• _ can't be accessed inside an initializer list.

class C {
  var _;
  var other;

  C(this._): other = _; // Error, cannot access `this`.
}

• The name _ can be used in the body, but this is a reference to the field, not the parameter.

class C {
  var _;

  C(this._) {
    print(_); // OK. Prints the field.
  }
}

• Error to have two initializing formals named _.

class C {
  var _;
  C(this._, this._); // Error.
}

• Error to have an occurrence of super._ as a declaration of a formal parameter in a constructor. This error also occurs in the case where the super parameter has an explicitly declared type and/or default value.

class B {
  final _;
  B(this._);
}

class C {
  C(super._); // Error.
}

• An extension type can have a parameter named _. This means that the representation variable is named _, and no formal parameter name is introduced into any scopes.

extension type E(int _) {
  int get value => _; // OK, the representation variable name is `_`.
  int get sameValue => this._; // OK.
}

• No error when a local declaration _ isn't used. (Avoid unused variable warnings.)
• The name _ is not introduced into the enclosing scope.
• Every kind of declaration named _ which is specified to be wildcarded is indeed accepted without compile-time errors