Add a fix to create an extension method

[bwilkerson]

Given a file containing the following (where c.dart contains the declaration of C):

import 'c.dart';

void f(C c) {
  var x = c.x;
  print(x);
}

It would be nice if the server offered a fix to create an extension method (getter). The extension could be added as an unnamed extension in the same file. If there's already an extension on the correct type it would be nice if the method were added to it rather than creating a new extension on the same type.

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/368502