usage: fix special handling of parameter initializer

[ajafff]

let foo; // this `foo` is unused
(bar = () => foo, foo) => bar;

Currently the foo parameter is marked as unused. Actually the outer variable foo is unused.

[ajafff]

Turns out to be a bug in newer versions of TypeScript: microsoft/TypeScript#22769

[ajafff]

Actually the above example is handled correctly by the compiler, but has a bug in this library:

let foo;
(bar = () => foo, foo) => bar; // initializer uses parameter `foo`

(bar = foo) => { // initializer uses outer variable `foo`
    var foo = 1;
}

[ajafff]

make sure the following still works correctly:

declare let foo: number;
function fn(bar: typeof foo) { // refers to the inner `foo`
    var foo = '';
}

[ajafff]

also make sure this weird clusterfuck either works as expected or is fixed upstream: microsoft/TypeScript#22825

[ajafff]

Another case:

let foo = 1;
(function({bar = foo}) {
  var foo = 2; // unused
  return bar; // returns 1
})({});