fix(list): fix role for list-item

Author: tinayuangao

src/lib/list/list.ts

@@ -119,7 +119,7 @@ export class MatListSubheaderCssMatStyler {}
   selector: 'mat-list-item, a[mat-list-item]',
   exportAs: 'matListItem',
   host: {
-    'role': 'listitem',
+    '[attr.role]': 'role',
     'class': 'mat-list-item',
     '(focus)': '_handleFocus()',
     '(blur)': '_handleBlur()',
@@ -134,6 +134,7 @@ export class MatListItem extends _MatListItemMixinBase implements AfterContentIn
     CanDisableRipple {
   private _lineSetter: MatLineSetter;
   private _isNavList: boolean = false;
+  role: string | null = null;
 
   @ContentChildren(MatLine) _lines: QueryList<MatLine>;
 
@@ -150,6 +151,7 @@ export class MatListItem extends _MatListItemMixinBase implements AfterContentIn
               @Optional() private _navList: MatNavList) {
     super();
     this._isNavList = !!_navList;
+    this.role = this._isNavList ? null : 'listitem';
   }
 
   ngAfterContentInit() {