minor fixes

Author: annieyw

src/cdk/tree/nested-node.ts

@@ -63,7 +63,8 @@ export class CdkNestedTreeNode<T> extends CdkTreeNode<T> implements AfterContent
     super(_elementRef, _tree);
     // The classes are directly added here instead of in the host property because classes on
     // the host property are not inherited with View Engine.
-    this._elementRef.nativeElement.classList.add('cdk-tree-node', 'cdk-nested-tree-node');
+    this._elementRef.nativeElement.classList.add('cdk-tree-node');
+    this._elementRef.nativeElement.classList.add('cdk-nested-tree-node');
   }
 
   ngAfterContentInit() {

src/cdk/tree/tree.ts

@@ -48,6 +48,7 @@ import {
   getTreeMultipleDefaultNodeDefsError,
   getTreeNoValidDataSourceError
 } from './tree-errors';
+import {coerceNumberProperty} from '@angular/cdk/coercion';
 
 /**
  * CDK tree component that connects with a data source to retrieve data of type `T` and renders
@@ -360,7 +361,7 @@ export class CdkTreeNode<T> implements FocusableOption, OnDestroy, OnInit {
 
   ngOnInit(): void {
     this._parentNodeAriaLevel = getParentNodeAriaLevel(this._elementRef.nativeElement);
-    this._elementRef.nativeElement.setAttribute('aria-level', String(this.level + 1));
+    this._elementRef.nativeElement.setAttribute('aria-level', `${this.level + 1}`);
   }
 
   ngOnDestroy() {
@@ -402,14 +403,14 @@ function getParentNodeAriaLevel(nodeElement: HTMLElement): number {
       return -1;
     }
   } else if (parent.classList.contains('cdk-nested-tree-node')) {
-    return Number(parent.getAttribute('aria-level')!);
+    return coerceNumberProperty(parent.getAttribute('aria-level')!);
   } else {
     // The ancestor element is the cdk-tree itself
     return 0;
   }
 }
 
 function isNodeElement(element: HTMLElement) {
-  const classList = element?.classList;
+  const classList = element.classList;
   return !(classList?.contains('cdk-nested-tree-node') || classList?.contains('cdk-tree'));
 }

src/material/tree/tree.ts

@@ -28,7 +28,8 @@ import {MatTreeNodeOutlet} from './outlet';
     // to be set in the host because:
     // if it is set as a @HostBinding it is not set by the time the tree nodes try to read the
     // class from it.
-    // the ElementRef is not available in the constructor so the class can't be applied directly.
+    // the ElementRef is not available in the constructor so the class can't be applied directly
+    // without a breaking constructor change.
     'class': 'mat-tree cdk-tree',
     'role': 'tree',
   },