change role input to always be treeitem

Author: annieyw

src/material/tree/node.ts

@@ -45,14 +45,14 @@ const _MatTreeNodeMixinBase: HasTabIndexCtor & CanDisableCtor & typeof CdkTreeNo
   host: {
     '[attr.aria-expanded]': 'isExpanded',
     '[attr.aria-level]': 'level + 1',
-    '[attr.role]': `'treeitem'`,
+    '[attr.role]': 'role',
     'class': 'mat-tree-node'
   },
   providers: [{provide: CdkTreeNode, useExisting: MatTreeNode}]
 })
 export class MatTreeNode<T> extends _MatTreeNodeMixinBase<T>
     implements CanDisable, HasTabIndex {
-  @Input() role: 'treeitem' | 'group' = 'treeitem';
+  @Input() role = 'treeitem';
 
   constructor(protected _elementRef: ElementRef<HTMLElement>,
               protected _tree: CdkTree<T>,
@@ -87,7 +87,7 @@ export class MatTreeNodeDef<T> extends CdkTreeNodeDef<T> {
   exportAs: 'matNestedTreeNode',
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.role]': `'treeitem'`,
+    '[attr.role]': 'role',
     'class': 'mat-nested-tree-node',
   },
   providers: [