[display] no expected-type completion for (Map) array-assignment

```haxe
class Main {
static function main() {
var x = new Map();
x[""] = |
}
}
```

no `{fields...}` 😢

Works with Array, didn't test with custom/non-multiType abstracts, but I suspect it's a general abstract operator overloading issue. Could we at least provide the expected type when there's only one `@:op([])`?