Improper @extends generates bad code with no error message

Input:

```js
/** @abstract */
class Base {
/** @return {string} */
foo() {}
other() { return 'other'; }
}

/** @extends {Base} */
class Derived {
foo() {
return 'derived';
}
}

/** @param {!Base} x */
function f(x) {
console.log(x.foo());
console.log(x.other());
}

f(new Derived());
```

Output:

```js
var a=new function(){};console.log("derived");console.log(a.a());
```

Note that `a.a()` doesn't exist. I think the code is in error in that it says `@extends` without adding any run-time extension, but in that case I think it should be rejected by the compiler.