js/libs/ember-1.5.1.js
function() {
        // Get the current value of the property
        newClass = this._classStringForProperty(binding);
        elem = this.$();

        // If we had previously added a class to the element, remove it.
        if (oldClass) {
          elem.removeClass(oldClass);
          // Also remove from classNames so that if the view gets rerendered,
          // the class doesn't get added back to the DOM.
          classNames.removeObject(oldClass);
        }

        // If necessary, add a new class. Make sure we keep track of it so
        // it can be removed in the future.
        if (newClass) {
          elem.addClass(newClass);
          oldClass = newClass;
        } else {
          oldClass = null;
        }
      }
function() {
        // Get the current value of the property
        newClass = this._classStringForProperty(binding);
        elem = this.$();

        // If we had previously added a class to the element, remove it.
        if (oldClass) {
          elem.removeClass(oldClass);
          // Also remove from classNames so that if the view gets rerendered,
          // the class doesn't get added back to the DOM.
          classNames.removeObject(oldClass);
        }

        // If necessary, add a new class. Make sure we keep track of it so
        // it can be removed in the future.
        if (newClass) {
          elem.addClass(newClass);
          oldClass = newClass;
        } else {
          oldClass = null;
        }
      }
Set up an observer on the context. If the property changes, toggle the class name.