No error for wrong overloads with computed properties

In the following example, the compiler yields only one error, though all the three cases are incorrect:

``` ts
class Foo {
    a();
    b() { }
}


class Bar {
    ["a"]();
    ["b"]() {}
}

class Baz {
    [Symbol.hasInstance]();
    [Symbol.iterator]() { }
}
```
