Auto complete on variable destruction can be improved

[FMorschel]

Originally opened at Dart-Code/Dart-Code#5379.

---

Is your feature request related to a problem? Please describe.
When you have code like this:

void foo(Iterable iterable) {
  if (iterable case List()) {
    
  }
}

When inside List(^) you get prompted by auto-complete for all proprieties the class has. But when you auto-complete, it simply fills the propriety name and not :var isEmpty which is a valid syntax for getting that value (in for in loops you also can do this but the var is then before the Type name). We already have the rename refactor for changing the variable name to a new one if we want to, but I feel like this would help more than harm most of the time.

Describe the solution you'd like
For it to auto-complete with :var before the propriety name (if you know what lints are available maybe we can look at the ones that ask for the final keyword or the type name) on if cases and without the var keyword on for in loops.

[FMorschel]

@srawlins can you think of any other case where we can have variable destruction like this? Did I forget anything?

[FMorschel]

I also thought about code like destructing records:

var (: myVar) = record;