Mat-expansion-panel bug

[petivagyok16]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Close 2 level deep child mat-expansion-panel-s when a parent mat-expansion-panel was closed.

What is the current behavior?

2 level deep Child mat-expansion-panel doesn't close but stays on the page

What are the steps to reproduce?

What is the use-case or motivation for changing an existing behavior?

It's bugged if we want to use more than 1 level deep expansion panel.

Which versions of Angular, Material, OS, TypeScript, browsers are affected?

"@angular/material": "^5.0.2",
"@angular/core": "^5.2.0",
Chrome 63
macOS 10.13.2 High Sierra

Is there anything else we should know?

No

[josephperrott]

Closing as duplicate of #8754

Will be fixed by #8820

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