Add warnings for incorrect use of goog.abstractMethod

Just spent some time on an issue that boiled down to:

```
/** @constructor */
example.Foo = goog.abstractMethod;

example.Foo.prototype.bar = goog.abstractMethod;
```

which should have been

```
/** @constructor */
example.Foo = function() {};

example.Foo.prototype.bar = goog.abstractMethod;
```

The compiler removed example.Foo entirely, so then a `goog.inherits(example.SubFoo, example.Foo)` call later on failed. We should warn for incorrect uses of goog.abstractMethod so this doesn't happen.