Create member fixes could do a better job of calculating record types

[FMorschel]

Repro:

void foo(String s) {
  var (:String name) = s.pattern;
}

The above doesn't know what the return type should be. We should look into this to see if we can infer ({String name,}) for the above and some other cases.

I'll eventually look into this one. If someone else wants to do that, please mention it in the comments. Thanks!

CC @bwilkerson @srawlins

[bwilkerson]

I'm guessing that we're getting the type of the left-hand side when it's a variable but just aren't handling patterns.

Also, note that the trailing comma isn't required for named fields because they aren't ambiguous, so the type should be ({String name}).

[FMorschel]

Yes, you are right, in all statements.

[FMorschel]

Here is the CL https://dart-review.googlesource.com/c/sdk/+/464782

I got confused and posted here the CL for #62092

[FMorschel]

Here is the WIP CL for this https://dart-review.googlesource.com/c/sdk/+/464784. Things should be basically working, I just want the CL for the above-mentioned issue to land so I can handle for-in with patterns too at the same time.