Prefer block/closure style in #class_eval
Prefer block/closure style in #class_eval

Opal doesn’t support class_eval with String (due to limited control
given by JavaScript `eval()` function) but works well with a block.

ruby/mspec,kachick/mspec,kachick/mspec,rubinius/mspec,eregon/mspec,ruby/mspec,elia/mspec