async class method not highlighted

This class

``` javascript
class ClassName {
  async methodName(arg1) {
  }
}
```

should have `async` keyword highlighted as `jsAsyncKeyword`. But it is not. It  just have `jsClassBlock`.

If I'm correct adding `jsAsyncKeyword` to [this line](https://github.com/pangloss/vim-javascript/blob/master/syntax/javascript.vim#L138) should fix this, because it works
for me.
