Incorrect branch coverage logic with parameter and destructuring defaults

[getify]

Take this code as a minimal test case:

function foo(x = 2) {
   console.log(x);
}

foo();

If you run Istanbul (1.0 alpha1) against it, you get a 100% passing report for all parts.

However, there's a branch that wasn't tested here, the implied non-default branch. The only branch tested is the invoking of the = 2, but there's no testing of what happens when an argument is passed affirmatively for the x parameter. Istanbul should complain that this implied branch isn't being tested.

To illustrate, let me show you the equivalent code pre-ES6:

function foo(x) {
   x = x !== undefined ? x : 2;
   console.log(x);
}

foo();

As you can clearly see, there's definitely two branches, and only one of them is getting tested. In fact, Istanbul will correctly report 50% branch coverage here.

Of course, the same reasoning goes for destructuring defaults:

var { x = 2 } = { };

Istanbul calls that 100% covered, when it should report the implied non-default branch not being covered.

var tmp = { };
var x = tmp.x !== undefined ? tmp.x : 2;

This equivalent is correctly reported as only 50% covered.