bug(tree): tree does not initialize when data is set in constructor

[meblum]

Reproduction

https://stackblitz.com/edit/mat-tree-flat-data-source-bug?file=src/app/tree-flat-overview-example.ts

Steps to reproduce:

1. Try to initialize the data in the constructor
2. Run the app

Expected Behavior

The MatTreeFlatDataSource accepts an optional argument initialData. I would expect the data sorce to be properly initialized when set in the constructor.

components/src/material/tree/data-source/flat-data-source.ts

Line 144 in e99ca0a

initialData: T[] = []) {

Actual Behavior

Tree renders as if data is empty

Environment

• Angular: 11
• CDK/Material: 11
• Browser(s): latest chrome
• Operating System (e.g. Windows, macOS, Ubuntu): windows

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