DDC prints nullable function types poorly

[nshahan]

DDC does not correctly parenthesize when printing a function type to show nullability of the function.

Given the dart:

main() {
  var foo = (int Function()? fn) => fn == null ? 0 : fn();
  print(foo.runtimeType);
}

The vm and dart2js prints:

((() => int)?) => int

while ddc prints:

(() => int?) => int