fix(autocomplete): clean up md-scroll-mask element on destroy

* added test

Fixes angular#7049
Fixes angular#7128

src/components/autocomplete/autocomplete.spec.js

@@ -365,6 +365,49 @@ describe('<md-autocomplete>', function() {
       element.remove();
     }));
 
+    it('should remove the md-scroll-mask on cleanup', inject(function($mdUtil, $timeout, $material) {
+      spyOn($mdUtil, 'enableScrolling');
+
+      var scope = createScope();
+      var template =
+          '<md-autocomplete' +
+          '   md-selected-item="selectedItem"' +
+          '   md-search-text="searchText"' +
+          '   md-items="item in match(searchText)"' +
+          '   md-item-text="item.display"' +
+          '   placeholder="placeholder">' +
+          '  <md-item-template>{{item.display}}</md-item-template>' +
+          '  <md-not-found>Sorry, not found...</md-not-found>' +
+          '</md-autocomplete>';
+      var element = compile(template, scope);
+      var ctrl = element.controller('mdAutocomplete');
+
+      $material.flushOutstandingAnimations();
+
+      // Focus our input
+      ctrl.focus();
+
+      // Set our search text to a value that we know doesn't exist
+      scope.searchText = 'somethingthatdoesnotexist';
+
+      // Run our initial flush
+      $timeout.flush();
+      waitForVirtualRepeat(element);
+
+      // Wait for the next tick when the values will be updated
+      $timeout.flush();
+
+      expect(ctrl.hidden).toBeFalsy();
+
+      // Make sure we wrap up anything and remove the element
+      $timeout.flush();
+      element.remove();
+      scope.$destroy();
+
+      // Should be hidden on once the scope is destroyed to ensure proper cleanup (like md-scroll-mask is removed from the DOM)
+      expect($mdUtil.enableScrolling).toHaveBeenCalled();
+    }));
+
     it('should ensure the parent scope digests along with the current scope', inject(function($timeout, $material) {
       var scope = createScope(null, {bang: 'boom'});
       var template =

src/components/autocomplete/js/autocompleteController.js

@@ -176,6 +176,10 @@ function MdAutocompleteCtrl ($scope, $element, $mdUtil, $mdConstant, $mdTheming,
    * Removes any events or leftover elements created by this controller
    */
   function cleanup () {
+    if(!ctrl.hidden) {
+      $mdUtil.enableScrolling();
+    }
+
     angular.element($window).off('resize', positionDropdown);
     if ( elements ){
       var items = 'ul scroller scrollContainer input'.split(' ');