function registerUpgradedCallbackInternal(jsClass, callback) {
	    var regClass = findRegisteredClass_(jsClass);
	    if (regClass) {
	      regClass.callbacks.push(callback);
	    }
	  }
Class constructor for dropdown MDL component.
Implements MDL component design pattern defined at:
https://github.com/jasonmayes/mdl-component-design-pattern

@constructor
@param {HTMLElement} element The element that will be upgraded.