definition.js

var MyClass = Y.Base.create('my-class', Y.Base, [], {
  _mergeProps: function (value, prop) {
      var cur = this.get(prop);
      if (Y.Lang.isUndefined(cur)) {
        this.reset(prop); // will rest to { test: true } for this instance instead of always using the static default values
      }
      return Y.merge(cur, value);
  }
}, {
  ATTRS: {
    dict: {
      value: {
        foo: 'bar',
        test: false
      },
      setter: '_mergeProps'
    }
  }
});

implementation.js

var instance = new MyClass({
  dict: {
     test: true
  }
});
instance.get('dict'); // { foo: 'bar', test: true }