Support generators in class body

I was getting errors about generators in ES6 classes.

This commit fixes that issue. It also adds two lines of positive tests to guard against regression.

For reference, generators in classes are in the latest ES6 draft spec and work well with 6to5.
I tried tripping this up with several additions: 

```js
    "  *[1]() { yield; }",
    "  static *[1]() { yield; }",
    "  * ['*']() { yield; }",
    "  static *['*']() { yield; }",
    "  * [('*')]() { yield; }",
    "  static *[('*')]() { yield; }",
```

None of which broke the patch, but should probably be added anyway.