Fixed negative flag

[CodeLenny]

Closes #10

[andrewvsk]

Also you can optimize your implementation:

Assertion.addMethod('emit', function(name, timeout) {
    timeout   = timeout || 1500;
    const obj = utils.flag(this, 'object'); // this._obj

    const assert = expr => {
         //                         .not = false                              .not = true
         this.assert(expr, `expected #{this} to emit '${name}'`, `expected #{this} to not emit '${name}'`);
    };

    return new Promise((resolve, reject) => {
        let done = false;

        obj.once(name, (...args) => {
            try {
                if (done) return;
                done = true;

                assert(true);   // OK | ERROR (.not = true),

                resolve(args);
            } catch (err) {
                reject(err);
            }
        });

        setTimeout(() => {
            try {
                if (done) return;
                done = true;

                assert(false); // ERROR | OK (.not = true),

                resolve();
            } catch (err) {
                reject(err);
            }
        }, timeout);
    });
});

'if (done)' - is unnecessary if on event clears timer. But with 'done' code looks more understandable.