Assist similar to `Split && condition` to work with `case when`

[FMorschel]

Today we have the following assits in this code:

void foo(List<int>? o) {
  if (o is List && o!.isEmpty) {} // Split && condition
}

I'm proposing we allow the same behaviour to work with case ... when so we split the when expression to an inner if.

CC @bwilkerson

[FMorschel]

Here is my suggested CL https://dart-review.googlesource.com/c/sdk/+/456061