bug(tree): Leaf nodes are not collapsed if top-level folder is collapsed

[helmbold]

Reproduction

The bug is reproducible with the flat tree example in the official documentation. What is the same as the respective Stackblitz example.

Steps to reproduce:

1. Expand "Vegetables" node and then "Green" and "Orange" nodes.
2. Collapse "vegetables"

Expected Behavior

All nodes under "Vegetables" should disappear (should be collapsed).

Actual Behavior

"Green" and "Orange" disappear, but the leaf nodes are still visible.

Expanded nodes:

After collapsing "Vegetables":

Environment

• Angular: 10.2.2
• CDK/Material: 10.2.2
• Browser(s): Chrome 85
• Operating System (e.g. Windows, macOS, Ubuntu): Windows

[mmalerba]

This looks like an issue for the cdk flat tree example. The material flat and nested tree examples work fine, and the cdk nested tree example works fine.

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