change isExpanded setter to private method

Author: annieyw

src/cdk/tree/tree.ts

@@ -346,7 +346,7 @@ export class CdkTreeNode<T> implements DoCheck, FocusableOption, OnDestroy, OnIn
     return this._tree.treeControl.isExpanded(this._data);
   }
 
-  set isExpanded(_expanded: boolean) {
+  private _setExpanded(_expanded: boolean) {
     this._expanded = _expanded;
     this._elementRef.nativeElement.setAttribute('aria-expanded', `${_expanded}`);
   }
@@ -370,7 +370,7 @@ export class CdkTreeNode<T> implements DoCheck, FocusableOption, OnDestroy, OnIn
     // TODO: move to host after View Engine deprecation
     this._elementRef.nativeElement.classList.add('cdk-tree-node');
     this.role = 'treeitem';
-    this.isExpanded = this.isExpanded;
+    this._setExpanded(this.isExpanded);
   }
 
   ngOnInit(): void {
@@ -384,7 +384,7 @@ export class CdkTreeNode<T> implements DoCheck, FocusableOption, OnDestroy, OnIn
     // It is not set using a @HostBinding because they sometimes get lost with Mixin based classes.
     // TODO: move to host after View Engine deprecation
     if (this.isExpanded != this._expanded) {
-      this.isExpanded = this.isExpanded;
+      this._setExpanded(this.isExpanded);
     }
   }
 

tools/public_api_guard/cdk/tree.d.ts

@@ -74,7 +74,6 @@ export declare class CdkTreeNode<T> implements DoCheck, FocusableOption, OnDestr
     get data(): T;
     set data(value: T);
     get isExpanded(): boolean;
-    set isExpanded(_expanded: boolean);
     get level(): number;
     get role(): 'treeitem' | 'group';
     set role(_role: 'treeitem' | 'group');