change aria-level binding to one-based

Author: annieyw

src/cdk/tree/tree.spec.ts

@@ -126,7 +126,7 @@ describe('CdkTree', () => {
         dataSource.addChild(data[0], true);
 
         const ariaLevels = getNodes(treeElement).map(n => n.getAttribute('aria-level'));
-        expect(ariaLevels).toEqual(['1', '2', '1', '1']);
+        expect(ariaLevels).toEqual(['2', '3', '2', '2']);
       });
 
       it('with the right data', () => {

src/cdk/tree/tree.ts

@@ -299,7 +299,7 @@ export class CdkTree<T> implements AfterContentChecked, CollectionViewer, OnDest
   exportAs: 'cdkTreeNode',
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.aria-level]': 'level',
+    '[attr.aria-level]': 'level + 1',
     '[attr.role]': 'role',
     'class': 'cdk-tree-node',
   },

src/material/tree/node.ts

@@ -44,7 +44,7 @@ const _MatTreeNodeMixinBase: HasTabIndexCtor & CanDisableCtor & typeof CdkTreeNo
   inputs: ['disabled', 'tabIndex'],
   host: {
     '[attr.aria-expanded]': 'isExpanded',
-    '[attr.aria-level]': 'level',
+    '[attr.aria-level]': 'level + 1',
     '[attr.role]': 'role',
     'class': 'mat-tree-node'
   },

src/material/tree/tree.spec.ts

@@ -72,7 +72,7 @@ describe('MatTree', () => {
         fixture.detectChanges();
 
         const ariaLevels = getNodes(treeElement).map(n => n.getAttribute('aria-level'));
-        expect(ariaLevels).toEqual(['0', '0', '0', '1']);
+        expect(ariaLevels).toEqual(['1', '1', '1', '2']);
       });
 
       it('with the right data', () => {