goog.abstractMethod vs @abstract

Since 08606a76, the compiler warn if abstract methods appear in concrete classes:

``` js
/**
* @abstract
* @constructor
*/
C = function() {
this.foo = -1;
};

/**
* @abstract
*/
C.prototype.add = function() {};
```

But it is not the case when `goog.abstractMethod` is used:

``` js
/**
* @constructor
*/
C = function() {
this.foo = -1;
};

/**
*
*/
C.prototype.add = goog.abstractMethod;
```

(and `goog.abstractMethod` doesn't require the constructor to be abstract.)