Turn `convert to getter` assist into `extension_type_declares_instance_field` fix

[FMorschel]

There is a Convert to getter assist for turning final fields into getters.

Also, for the following code:

extension type A(int i) {
  final int a;
}

We get:

Extension types can't declare instance fields.
Try replacing the field with a getter. extension_type_declares_instance_field

I think we should make that assist a possible fix for this error.

I'll work on this.

[FMorschel]

https://dart-review.googlesource.com/c/sdk/+/402622

[FMorschel]

@bwilkerson, on the CL you mentioned your opinion on offering this assist for cases like final int foo; where making the conversion would generate invalid code:

I think it's better to not offer the assist when the result will be invalid code. Some users assume that if we're suggesting a change that it must be a good change, and producing code with diagnostics tends to decrease confidence in our tools.

In this case, I intended to lay the structural change only, similar to calling Create function on the following code would generate an int foo() {} which has a warning (because of no return) but we do it anyway:

int value = foo();

Pointing it out so you know where my thoughts come from.

[FMorschel]

Also for the following code I created class A {} and I used Create getter on a.getter and this is the current output (basically what I suggested):

void foo() {
  var a = A();
  int value = a.getter;
}

class A {
  int get getter => null;   // Create getter output
}

So my point for keeping that option is relying on the options we already have today. I could also your your input @scheglov since you seemed to have no problem with this suggestion on the CL.

[bwilkerson]

I replied on the CL, but I agree that what you were proposing is fine.

[scheglov]

My opinion is that generating int get getter => null; is fine. It will report an error, but this itself will help to see the code that should be updated to provide an actual value.

[FMorschel]

Just as a note, once I solve #59847, I'll come back to this CL/issue.

[FMorschel]

Just posted a new patchset, would love new reviews here. Thanks!