[private-named-parameters] Dartdoc Support

[munificent]

This issue tracks the work needed to support "Private Named Parameters" in Dartdoc.

This feature does impact Dartdoc in that we want a private named parameter to be documented as its public name. Given a class like:

class C {
  int _x;
  C({required this._x});
}

Then the dartdoc for the constructor should look something like:

C({required int x});

[lrhn]

Also, all visible textual references to the parameter in the actual generated DartDoc should use the public name, so

class Baz {
  final int foo, _bar;
  /// Creates `Baz` from [foo] and [_bar].
  new(this.foo, {this._bar}) : asset(_bar != 0);
}

will generate a documentation of:

Creates <code>Baz</code> from
<code>foo</code> and <code>bar</code>.

(It would be nice if DartDoc did the same thing for positional private initializing or declaring parameters, otherwise I'll still have to do (int foo): _foo = foo;. That needs to check if there is another parameter which has the corresponding make already, but otherwise it would be the same.)

[sigurdm]

@johnniwinther this sounds like the analyzer api should provide a "external name" of parameters somehow?

[johnniwinther]

Yes. To the extent that we use AST nodes for printing the constructor parameters we need this. The element should show the public name in all cases.