[js] type inference breaks calls due to missing $bind

```haxe
class Main {
static function main() {
foo(new Main());
}

static function foo(main) {
bar(main.call);
}

static function bar(f:() -> Int) {
trace(f());
}

function new() {}

var field = 0;

public function call():Int {
return field;
}
}
```

Crashes with:

```
return this.field;
^

TypeError: Cannot read property 'field' of undefined
```

With the above `foo` looks like this:

```haxe
Main.foo = function(main) {
Main.bar(main.call);
};
```

Explicitly type-hinting `main` in `foo` as `main:Main` works since a `$bind` ends up being generated:

```haxe
Main.foo = function(main) {
Main.bar($bind(main,main.call));
};
```

It also works with `main:{function call():Int;}`, which is why @nadako mentioned this is related to some previous discussions about whether unification between functions and function-typed vars in structures should be allowed.