Cannot instantiate abstract class

[Repro][1]

The receiver type for constructor functions is covariant, but when I pass a constructor for an abstract class into a function parameter and then try to instantiate it within the function, the compiler assumes I'm trying to instantiate exactly the abstract base class, rather than admitting the possibility that the instance variable is a concrete subclass.

```js
/** @abstract */ class Foo {}
class Bar extends Foo {}

/** @param {function(new:Foo)} ctor */
function f(ctor) {
new ctor();
}

f(Bar);
```

gives the error

```
WARNING - cannot instantiate abstract class
new ctor();
^^^^^^^^^^
```

This error is fine when the constructor is known to be for a specific class, but when it's possibly for any subclass, it no longer makes any sense.

[1]: https://closure-compiler-debugger.appspot.com/#input0%3D%252F**%2520%2540abstract%2520*%252F%2520class%2520Foo%2520%257B%257D%250Aclass%2520Bar%2520extends%2520Foo%2520%257B%257D%250A%250A%252F**%2520%2540param%2520%257Bfunction(new%253AFoo)%257D%2520ctor%2520*%252F%250Afunction%2520f(ctor)%2520%257B%250A%2520%2520new%2520ctor()%253B%250A%257D%250A%250Af(Bar)%253B%26input1%26conformanceConfig%26externs%3Dfunction%2520Symbol()%2520%257B%257D%250A%252F**%2520%2540constructor%2520%2540return%2520%257B!Object%257D%2520*%252F%2520function%2520Object(%252F**%2520*%253D%2520*%252F%2520opt_arg)%2520%257B%257D%250AObject.create%2520%253D%2520function(%252F**%2520...%253F%2520*%252F%2520var_args)%2520%257B%257D%253B%250AObject.getOwnPropertyDescriptor%2520%253D%2520function(%252F**%2520...%253F%2520*%252F%2520var_args)%2520%257B%257D%253B%250AObject.defineProperty%2520%253D%2520function(%252F**%2520...%253F%2520*%252F%2520var_args)%2520%257B%257D%253B%250AObject.setPrototypeOf%2520%253D%2520function(%252F**%2520...%253F%2520*%252F%2520var_args)%2520%257B%257D%253B%250AObject.prototype.constructor%253B%250A%252F**%2520%2540constructor%2520*%252F%2520function%2520Function(%252F**%2520...*%2520*%252F%2520var_args)%2520%257B%257D%250AFunction.prototype.apply%2520%253D%2520function(%252F**%2520...%253F%2520*%252F%2520var_args)%2520%257B%257D%253B%250A%252F**%2520%2540constructor%2520%2540return%2520%257B!TypeError%257D%2520*%252F%2520function%2520TypeError(%252F**%2520*%253D%2520*%252F%2520opt_arg0%252C%252F**%2520*%253D%2520*%252F%2520opt_arg1%252C%252F**%2520*%253D%2520*%252F%2520opt_arg2)%2520%257B%257D%250A%26refasterjs-template%26CHECK_SYMBOLS%3D1%26MISSING_PROPERTIES%3D1%26TRANSPILE%3D1%26CHECK_TYPES%3D1%26CLOSURE_PASS%3D1%26PRESERVE_TYPE_ANNOTATIONS%3D1%26PRETTY_PRINT%3D1