Error when @override method takes a type union as a parameter

As part of an attempt to whittle down the remaining errors in `google/elemental2` builds I tried to define the standardized `Document.prototype.write` in an extern accessible via Elemental2.

The extern looks something like:

```javascript
/**
* @param {!TrustedHTML|string} text
* @return {undefined}
* @see https://html.spec.whatwg.org/multipage/dynamic-markup-insertion.html#dom-document-write
*/
Document.prototype.write = function(text) {};
```

This is overridden in `w3c_dom2.js` with

```javascript
/**
* @param {!TrustedHTML|string} text
* @return {undefined}
* @see http://www.w3.org/TR/2000/CR-DOM-Level-2-20000510/html.html#ID-75233634
* @override
*/
HTMLDocument.prototype.write = function(text) {};
```

Unfortunately `jsinterop-generator` generates the overlay methods for union types in `Document` class like

```java
@JsOverlay
public final void write(TrustedHTML text) {
write(Js.uncheckedCast(text));
}
```

and attempts to define a similar but different overlay in `HTMLDocument` like

```java
@JsOverlay
public final void write(TrustedHTML text) {
write(Js.uncheckedCast(text));
}
```

There does not seem to get the overriding method to use the union type and thus the overlay methods declared by the overridden method. Thus a compile error.