fix(cdk/tree): also update flat level accessor example

Author: BobobUnicorn

src/components-examples/cdk/tree/cdk-tree-flat-level-accessor/cdk-tree-flat-level-accessor-example.ts

@@ -20,6 +20,8 @@ export class CdkTreeFlatLevelAccessorExample {
   getParentNode(node: FlatFoodNode) {
     const nodeIndex = FLAT_DATA.indexOf(node);
 
+    // Determine the node's parent by finding the first preceding node that's
+    // one level shallower.
     for (let i = nodeIndex - 1; i >= 0; i--) {
       if (FLAT_DATA[i].level === node.level - 1) {
         return FLAT_DATA[i];
@@ -29,14 +31,9 @@ export class CdkTreeFlatLevelAccessorExample {
     return null;
   }
 
-  shouldRender(node: FlatFoodNode) {
-    let parent = this.getParentNode(node);
-    while (parent) {
-      if (!parent.isExpanded) {
-        return false;
-      }
-      parent = this.getParentNode(parent);
-    }
-    return true;
+  shouldRender(node: FlatFoodNode): boolean {
+    // This node should render if it is a root node or if all of its ancestors are expanded.
+    const parent = this.getParentNode(node);
+    return !parent || (!!parent.isExpanded && this.shouldRender(parent));
   }
 }