Support use of `this` keyword in static ES6 methods

It's very common (and safe) to use the `this` keyword in a static member definition:

```js
class Foo {
  static get baz() { return 'baz'; }

  static bar() {
    console.log(this.baz);
  }
}
```

We should replace the `this` keyword with a reference to the class name.