Arrow functions with object-like block bodies are not as expected ES style code

[STRd6]

x => {
  f(y)
}

Actual:

x(() => {
  f: f(y);
});

ES Compat:

x(() => {
  return f(y);
});

This is not really a bug but more of a tradeoff that we've discussed before. In any case it probably needs an es-compat flag.