Add dart fix support for moved classes

[Piinks]

We would like to supply fixes that can change a class name based on the method being called.

For example:

Scaffold.of(context).showSnackBar(SnackBar(content: Text('Snack')));

should be migrated to

ScaffoldMessenger.of(context).showSnackBar(SnackBar(content: Text('Snack')));

The is not specific to the same method being moved across classes, as we would want to be able to identify and compose fixes that can feasibly migrate classA.methodA to classB.methodB, or constructors too.

Another example of this would be going from this:

BlackListingTextInputFormatter()

to this:

FilteringTextInputFormatter.deny()

Not sure if that example should be a different use case, if so we can break it out into a separate issue.

[craiglabenz]

I second this feature request, as it would open up a lot of very powerful migrations.

[bwilkerson]

I think the following new kind of change would satisfy the requirements expressed here:

---

replacedBy

A replaced by change indicates that the specified element was replaced by another element. It has two keys: kind, and newElement.

The newElement key is required. The value of the newElement key is an element.

---

I think this would allow you to specify that the static method of in the class Scaffold was replaced by the static method of in the class ScaffoldMessenger, or that the constructor BlackListingTextInputFormatter was replaced by the constructor FilteringTextInputFormatter.deny.

Can you think of any other information you might want to have supported by such a change?

[craiglabenz]

As a Dartfix beginner, I'm going to ask a beginner question:

Could this be combined with Dartfixes for any parameters that were renamed between the old and new methods / constructors?

AKA, turning this:

ClassA({required int count})

to this

ClassB({required int limit})

[bwilkerson]

Good question. I believe that the answer ought to be 'yes', that it ought to be possible to specify changes to the parameters just like you can with a rename change. But it's possible that there are implementation problems that I'm not currently aware of that would prevent that for some reason, so I'm hesitant to promise that it will be possible.

[pq]

See also: https://dart-review.googlesource.com/c/sdk/+/214068 for a place where replacedBy could come in handy in migrated some SDK constant references. (There we'd like to change references to top-level variables (like READ) to class constants (like FileMode.read).)

/fyi @natebosch

[bwilkerson]

I have committed https://dart-review.googlesource.com/c/sdk/+/221900, which includes a limited version of a replaceBy change. I believe that what's currently implemented is sufficient for both the use cases above and for the SDK use case. I'll update the documentation to include the new change, hopefully today.

I'm going to close this issue in preference of having new issues for any enhancement requests.

[pq]

@bwilkerson: maybe cc @natebosch when you update the docs since this ought to get them unblocked?