Dart argument definition test deprecated?

On Dart 1.0.0, I just tried:

class MyClass {
    int x;
    bool b;

    MyClass(int x, [bool b = true]) {
        if(?b) {
            // ...
        }
    }
}

And am getting a compiler error on the ?b part:

The argument definition test ('?' operator) has been deprecated

So what's the "new" way of testing for whether or not an argument was supplied?