es6 swallowing console errors

Question

I'm using ES6 in my project and am finding that no console errors are being shown. I'm using Gulp with Babel (require('gulp-babel');) to transpile the ES6.

Take the following example: I instantiate a class which extends a parent, with method method1() then call a method that doesn't exist super.method2() in the child.

The program stops running but I get no errors. Why is this?

Other file:

...
API.Quest.getQuest(1).then((quest) => {
    this.quest = new Quest(quest);
    console.log('DONE'); // never gets here
}, (err) => {
    console.log('ERR', err);
});

Classes:

class Event {
    constructor(args, childEventClass, childEventClassName) {
        this.name = args.name;
        this.id = args.id;
        this.children = args[childEventClassName].map(eventObj => new childEventClass(eventObj));
        ...
    }

    method1() {
        console.log('Called parent method 1');
    }
}

class Quest extends Event {
    constructor(args) {
        super(args, Task, 'tasks');
        super.method2();
    }
};

class Task extends Event {
    ...
};

Gulp:

const babel = require('gulp-babel');
...
gulp.task('scripts', function() {
    return gulp.src([
        'js/**/*.js',
    ])
        .pipe(babel({
            presets: ['es2015']
        }))
        .pipe(concatJs('main.js'))
        .pipe(gulp.dest('build/js'));
});

---

Edit 2:

    API.Quest.getQuest(1).then((quest) => {
        // ... 
    }, (err) => {
        console.error('REJECT ERR', err);
        // Catch rejected promises
    }).catch((err) => {
        // Catch ?
        console.error('CATCH ERR', err);
    });