code: function(expression, onChange, callbackContext) {
      var observer = this._observations.createObserver(expression, onChange, callbackContext || this);
      this._observers.push(observer);
      if (this.observersEnabled) observer.bind(this._context);
      return observer;
    }
docs: Watch this object for changes in the value of the expression
  @param {String} expression The expression to observe
  @param {Function} onChange The function which will be called when the expression value changes
  @return {Observer} The observer created