Static methods that reference `this` don't work correctly with CollapseProperties

```js
class Base {
/** @return {Function} */
static foo() { return this; }
}

Base.foo();
```

With CollapseProperties on, this compiles to

```js
function Base$foo() { return this; }
Base$foo();
```

This returns `undefined` rather than `Base`. We should either warn upon collapsing a static method with a reference to `this`, or else automatically add `@nocollapse` upon transpiling such a method.