Backbone.sync = function(method, model, options) {
  var data = model.toJSON()
  setTimeout(function() {
    options.success(_.extend({ 'id': 1, 'b': 2, 'c': true }, data), {})
  }, 10)
};

var A = Backbone.Model.extend({})
  , a = new A()

a.fetch({
  success: function() {
    a.on('change', function(model, evt) { console.log(_.keys(evt.changes)) })
    a.on('change:id', function(model, data) { console.log('change:id: ' + data) })
  }
})

setTimeout(function() {
  console.log('no wait')
  a.save({ 'b': 6 })
}, 100)

setTimeout(function() {
  console.log('wait: true')
  a.save({ 'b': 8 }, { wait: true })
}, 200)