build: test failure in example (#32996)

Fixes that a test is failing after some recent changes.

Author: crisbeto

src/components-examples/material/tree/tree-harness/tree-harness-example.html

@@ -6,7 +6,11 @@
     {{node.name}}
   </mat-tree-node>
   <!-- This is the tree node template for expandable nodes -->
-  <mat-tree-node *matTreeNodeDef="let node;when: hasChild" matTreeNodePadding matTreeNodeToggle
+  <mat-tree-node
+    *matTreeNodeDef="let node; when: hasChild"
+    matTreeNodePadding
+    isExpandable
+    matTreeNodeToggle
     [cdkTreeNodeTypeaheadLabel]="node.name">
     <button matIconButton matTreeNodeToggle [attr.aria-label]="'Toggle ' + node.name">
       <mat-icon class="mat-icon-rtl-mirror">
@@ -15,4 +19,4 @@
     </button>
     {{node.name}}
   </mat-tree-node>
-</mat-tree>
+</mat-tree>