feat(cdk/tree): report an error when the API consumer tries to expand a non-expandable node.

Author: BobobUnicorn

src/cdk/tree/tree-errors.ts

@@ -54,3 +54,15 @@ export function getMultipleTreeControlsError() {
 export function getTreeControlNodeTypeUnspecifiedError() {
   return Error(`The nodeType was not specified for the tree.`);
 }
+
+/**
+ * Returns an error to be thrown when a node is attempted to be expanded or collapsed when
+ * it's not expandable.
+ * @docs-private
+ */
+export function getNodeNotExpandableError() {
+  return Error(
+    `The node that was attempted to be expanded or collapsed is not expandable; you might ` +
+      `need to provide 'isExpandable'.`,
+  );
+}

src/cdk/tree/tree.ts

@@ -61,6 +61,7 @@ import {CdkTreeNodeDef, CdkTreeNodeOutletContext} from './node';
 import {CdkTreeNodeOutlet} from './outlet';
 import {
   getMultipleTreeControlsError,
+  getNodeNotExpandableError,
   getTreeControlMissingError,
   getTreeControlNodeTypeUnspecifiedError,
   getTreeMissingMatchingNodeDefError,
@@ -1097,12 +1098,18 @@ export class CdkTreeNode<T, K = T> implements OnDestroy, OnInit, TreeKeyManagerI
 
   /** Collapses this data node. Implemented for TreeKeyManagerItem. */
   collapse(): void {
+    if (typeof ngDevMode === 'undefined' || (ngDevMode && !this._isExpandable())) {
+      throw getNodeNotExpandableError();
+    }
     this._tree.collapse(this._data);
     this.expandedChange.emit(this.isExpanded);
   }
 
   /** Expands this data node. Implemented for TreeKeyManagerItem. */
   expand(): void {
+    if (typeof ngDevMode === 'undefined' || (ngDevMode && !this._isExpandable())) {
+      throw getNodeNotExpandableError();
+    }
     this._tree.expand(this._data);
     this.expandedChange.emit(this.isExpanded);
   }