Support extending a mixin application expression directly

@azakus has talked to some folks about this offline but I wanted to make an issue to link to / track.

Currently to support ES6 class mixin, one must do something like:

```js

// declaration

/**
* @interface
*/
function TimedInterface(){}
/** @type {number} */
TimedInterface.prototype.time;

/**
* @template T
* @param {T} mixin ES6 class expression mixin to wrap
*/
function Timed(superclass) {
return class extends superclass {
constructor {
/** @type {number} */
this.time = 0;
}
}
};

// use

/**
* @implements {TimedInterface}
* @extends {Foo}
*/
const TimedFoo = Timed(Foo);

class Bar extends TimedFoo {};
```

There are two suboptimal parts to this. The first is that we need to manually duplicate the interface of the `Timed` class in TimedInterface. The second is that we can't write `class Bar extends Timed(Foo) {}` because extending an expression isn't supported.