When the same operator is used consecutively, support to append the operatior at the end

now, dart support
```
var lst = [
a,
b,
];
```

but not supprt
```
var lst =
a &&
b &&;
```

or

```
if ( a &&
b &&) {
.....
}
```

I think this feature is useful.

Review the consecutive-operator examples in the issue, including list literals and multiline && expressions. Determine the intended grammar change and document the affected language behavior; done means the proposal has a clear resolution for each example.

Written by the indexing model from the issue text.