fix(checkbox): ngModel intial value used correctly with events

Fixes #1550.

src/components/checkbox/checkbox.js

@@ -6,9 +6,8 @@
  * @name material.components.checkbox
  * @description Checkbox module!
  */
-angular.module('material.components.checkbox', [
- 'material.core'
-])
+angular
+ .module('material.components.checkbox', ['material.core'])
  .directive('mdCheckbox', MdCheckboxDirective);
 
 /**
@@ -78,7 +77,6 @@ function MdCheckboxDirective(inputDirective, $mdInkRipple, $mdAria, $mdConstant,
 
  return function postLink(scope, element, attr, ngModelCtrl) {
  ngModelCtrl = ngModelCtrl || $mdUtil.fakeNgModel();
- var checked = false;
  $mdTheming(element);
 
  if (attr.ngChecked) {
@@ -112,15 +110,14 @@ function MdCheckboxDirective(inputDirective, $mdInkRipple, $mdAria, $mdConstant,
  if (element[0].hasAttribute('disabled')) return;
 
  scope.$apply(function() {
- checked = !checked;
- ngModelCtrl.$setViewValue(checked, ev && ev.type);
+ // Toggle the checkbox value...
+ ngModelCtrl.$setViewValue( !ngModelCtrl.$viewValue, ev && ev.type);
  ngModelCtrl.$render();
  });
  }
 
  function render() {
- checked = ngModelCtrl.$viewValue;
- if(checked) {
+ if(ngModelCtrl.$viewValue) {
  element.addClass(CHECKED_CSS);
  } else {
  element.removeClass(CHECKED_CSS);