Formatter drops comment before comma in collection literal

Here's a minimal repro of an issue found in a larger piece of code. If you format:

```dart
var x = [
  1 // Comment 1.
  ,
  2 // Comment 2.
];
```

It outputs:

```dart
var x = [
  1,
  2, // Comment 2.
];
```

It seems to only have this bug if the first comment is *before* the comma (which is weird) *and* there is a second comment.