Use arrow functions when self = this is declared in parent scope

[eventualbuddha]

For example:

var self = this;
return function() {
  return self;
}

can become:

return () => this;

The usual caveats apply:

• referencing this inside the function disqualifies it (unless it's also a .bind(this) function?)
• referencing arguments inside the function disqualifies it (unless we can rewrite using rest params)