Add an example of generating an anonymous closure

[eseidel]

This fixes dart-lang/tools#1123.

[eseidel]

Amusingly if you leave off the ..body for Method it just produces '(a)' which is probably a separate bug. Also Code('') produces '{ }', note the space, which may also be a bug. 🤷

[eseidel]

I have no ability to merge, please do. :)

[natebosch]

if you leave off the ..body for Method it just produces '(a)' which is probably a separate bug

Yeah I do think we should fallback to an empty body. I'm not sure if there are other values that result in invalid code when omitted - we probably only cover the very common omissions.

note the space, which may also be a bug.

We don't make guarantees about whitespace in the output, but I can check it that's an easy space to omit.

[natebosch]

Thanks!