@final not checked when method is overridden by setting prototype

The docs imply that `@final` on a class method makes it an error to override that method in a subclass:

``` js
/** @constructor */
function MyClass() {}

/** @final */
MyClass.prototype.method = function() {};

/**
* @constructor
* @extends {MyClass}
*/
function MyChildClass() {}

MyChildClass.prototype = {
__proto__: MyClass,
/** @override */
method: function() {}, // should fail
};
```

But this doesn't error. The method has to be set separately:

``` js
MyChildClass.prototype.method = function() {}; // does fail
```

for closure-compiler to notice this mistake.