Array.join() and -dce full issue

https://try.haxe.org/#0Dc93

```haxe
class Foo {
public function new() {
}

public function toString()
return Std.string(Std.random(100) + 100);
}

class Test {
static function main() {
// trace(Std.string(new Foo()));
var foos: Array = [new Foo(), new Foo(), new Foo()];
trace(foos.join('/'));
}
}
```

Results in
```
[object Object]/[object Object]/[object Object]
```

If i uncomment the first trace `trace(Std.string(new Foo()));` i get the expected output (of some random numbers).

```
170
182/170/178
```

Is this expected behavior with DCE, I certainly was surprised by it?