[Declaring Constructors] Navigation with `primary_constructor` fragments

[FMorschel]

While reading dart-lang/language#4496 I came up with a question regarding navigation. Consider:

class C(final int x, final int y) {}

class D(super.x, super.y) extends C {
  @metadata
  this: assert(x != y) {
    print(x);
  }
}

I don't think there is (or that it makes any sense to have) a rule in the proposed syntax about the init block being the first thing in the class body (CC @eernstg @lrhn, can either of you confirm this, please? Thanks).

Now I was thinking about a similar thing to what was proposed in #54742 for navigation. Where we could display a "button" of sorts to navigate between the two. Especially if the init block is moved to the bottom of the class.

We can, of course, wait for this syntax to be fully defined before we start doing anything around here. This is simply so we don't forget to raise this question.

CC @DanTup @bwilkerson

[eernstg]

True, there is no rule that makes it an error for the body part of a primary constructor to be declared at any particular location in the class/enum/extension-type body. It will be a matter of style where to put it. However, I'd expect a position near the top of the body to be preferred, possibly supported by lints.