fix(cdk-tree): initialize cdk-tree component datasource in the constructor (#15)

Due to how class property initialization works in ES2022, we cannot access shorthand properties that are initialized via the constructor outside the constructor. Therefore, we need to move the initialization of some properties inside the constructor.

See #15 (comment) for more details.

Author: jzolnowski

src/app/angular/cdk-tree/cdk-tree.component.ts

@@ -1,4 +1,4 @@
-import { ArrayDataSource } from '@angular/cdk/collections';
+import { ArrayDataSource, DataSource } from '@angular/cdk/collections';
 import { FlatTreeControl } from '@angular/cdk/tree';
 import { Component } from '@angular/core';
 import { ExampleFlatNode } from './cdk-tree.model';
@@ -11,14 +11,17 @@ import { CdkTreeService } from './cdk-tree.service';
 })
 export class CdkTreeComponent {
   static selector = 'appCdkTree';
-  private readonly treeData: ExampleFlatNode[] = this.cdkTreeService.getTreeData();
-  treeControl = new FlatTreeControl<ExampleFlatNode>(
+  private readonly treeData: ExampleFlatNode[];
+  readonly treeControl = new FlatTreeControl<ExampleFlatNode>(
     node => node.level,
     node => node.expandable
   );
-  dataSource = new ArrayDataSource(this.treeData);
+  readonly dataSource: DataSource<ExampleFlatNode>;
 
-  constructor(private cdkTreeService: CdkTreeService) {}
+  constructor(private cdkTreeService: CdkTreeService) {
+    this.treeData = this.cdkTreeService.getTreeData();
+    this.dataSource = new ArrayDataSource(this.treeData);
+  }
 
   hasChild = (_: number, node: ExampleFlatNode) => node.expandable;