Non-local return in a block in a define_method body raises LJE when called

Ran into this running a Dashing app, based on Sinatra, via Puma+Rack. Here's the simplest reproduction:

``` ruby
class Foo
  define_method(:foo) { loop { return "foo" } }
end

Foo.new.foo
```

When the foo method is called, it will raise LocalJumpError.
