fix(cdk-tree): fix crash in test cases with max call stack

Fix crash in test cases " An error was thrown in afterAll. RangeError: Maximum call stack size exceeded."

example: https://app.circleci.com/pipelines/github/angular/components/58166/workflows/cc9e4f5d-2182-44a5-89a6-56f93e196843/jobs/506030

Fix crash when happened in expandAll and collapseAll methods. Use direct
value of `_flattenedNodes` rather than subscribing to avoid unnecessary
subscription. Using the value directly is safe because `_flattenedNodes`
is a synchronous cache.

src/cdk/tree/tree.ts

@@ -643,11 +643,9 @@ export class CdkTree<T, K = T>
       this.treeControl.expandAll();
     } else if (this._expansionModel) {
       const expansionModel = this._expansionModel;
-      this._getAllNodes()
-        .pipe(takeUntil(this._onDestroy))
-        .subscribe(children => {
-          expansionModel.select(...children.map(child => this._getExpansionKey(child)));
-        });
+      expansionModel.select(
+        ...this._flattenedNodes.value.map(child => this._getExpansionKey(child)),
+      );
     }
   }
 
@@ -657,11 +655,9 @@ export class CdkTree<T, K = T>
       this.treeControl.collapseAll();
     } else if (this._expansionModel) {
       const expansionModel = this._expansionModel;
-      this._getAllNodes()
-        .pipe(takeUntil(this._onDestroy))
-        .subscribe(children => {
-          expansionModel.deselect(...children.map(child => this._getExpansionKey(child)));
-        });
+      expansionModel.deselect(
+        ...this._flattenedNodes.value.map(child => this._getExpansionKey(child)),
+      );
     }
   }
 

src/material/tree/testing/tree-harness.spec.ts

@@ -227,7 +227,7 @@ interface ExampleFlatNode {
         {{node.name}}
       </mat-tree-node>
       <!-- This is the tree node template for expandable nodes -->
-      <mat-tree-node *matTreeNodeDef="let node;when: flatTreeHasChild" matTreeNodePadding>
+      <mat-tree-node *matTreeNodeDef="let node;when: flatTreeHasChild" matTreeNodePadding isExpandable>
         <button matTreeNodeToggle>
           Toggle
         </button>
@@ -240,7 +240,7 @@ interface ExampleFlatNode {
         {{node.name}}
       </mat-tree-node>
       <!-- This is the tree node template for expandable nodes -->
-      <mat-nested-tree-node *matTreeNodeDef="let node; when: nestedTreeHasChild">
+      <mat-nested-tree-node *matTreeNodeDef="let node; when: nestedTreeHasChild" isExpandable>
         <button matTreeNodeToggle>
           Toggle
         </button>