yield doesn't work inside a block

Ruby:

``` ruby
>> def lol(&block); [1].each {|v| yield v }; end; lol { |v| puts v.inspect }
1
=> [1]
```

Opal:

``` ruby
> bundle exec opal -e 'def lol(&block); [1].each {|v| yield v }; end; lol { |v| puts v.inspect }'

```
