perf(tree): leaking reference through mostRecentTreeNode (#12334)

Along the same lines as #12269. Clears out the `mostRecentTreeNode` once the last tree node is destroyed, in order to avoid a memory leak.

src/cdk/tree/tree.spec.ts

@@ -17,7 +17,7 @@ import {TreeControl} from './control/tree-control';
 import {FlatTreeControl} from './control/flat-tree-control';
 import {NestedTreeControl} from './control/nested-tree-control';
 import {CdkTreeModule} from './index';
-import {CdkTree} from './tree';
+import {CdkTree, CdkTreeNode} from './tree';
 import {getTreeControlFunctionsMissingError} from './tree-errors';
 
 
@@ -35,6 +35,20 @@ describe('CdkTree', () => {
     }).compileComponents();
   }
 
+  it('should clear out the `mostRecentTreeNode` on destroy', () => {
+    configureCdkTreeTestingModule([SimpleCdkTreeApp]);
+    const fixture = TestBed.createComponent(SimpleCdkTreeApp);
+    fixture.detectChanges();
+
+    // Cast the assertions to a boolean to avoid Jasmine going into an
+    // infinite loop when stringifying the object, if the test starts failing.
+    expect(!!CdkTreeNode.mostRecentTreeNode).toBe(true);
+
+    fixture.destroy();
+
+    expect(!!CdkTreeNode.mostRecentTreeNode).toBe(false);
+  });
+
   describe('flat tree', () => {
     describe('should initialize', () => {
       let fixture: ComponentFixture<SimpleCdkTreeApp>;

src/cdk/tree/tree.ts

@@ -290,7 +290,7 @@ export class CdkTree<T>
     'class': 'cdk-tree-node',
   },
 })
-export class CdkTreeNode<T>  implements FocusableOption, OnDestroy {
+export class CdkTreeNode<T> implements FocusableOption, OnDestroy {
   /**
    * The most recently created `CdkTreeNode`. We save it in static variable so we can retrieve it
    * in `CdkTree` and set the data to it.
@@ -328,6 +328,12 @@ export class CdkTreeNode<T>  implements FocusableOption, OnDestroy {
   }
 
   ngOnDestroy() {
+    // If this is the last tree node being destroyed,
+    // clear out the reference to avoid leaking memory.
+    if (CdkTreeNode.mostRecentTreeNode === this) {
+      CdkTreeNode.mostRecentTreeNode = null;
+    }
+
     this._destroyed.next();
     this._destroyed.complete();
   }