make some changes to js_ast to improve output

[sigmundch]

Some that come to mind:

• change \n printing to blocks, so that if(foo) return bah; can be a single line
• add newlines at the end of classes (I think the separation helps)

[jmesserly]

regarding:

add newlines at the end of classes (I think the separation helps)

I tried this, it looked a little off because other top level stuff doesn't necessarily have spacing. we could do a newline before and after, though. Another idea I had was to use ES6 modules, and then they can have better spacing between top level code.

[jmesserly]

moved to #425