disabled attribute is partially working on mat-button-toggle-group

[lokeshdaiya]

Bug, feature request, or proposal:

After disabling mat-button-toggle-group its still selectable.

What is the expected behavior?

Its should be completely disabled.

What is the current behavior?

When disabled attribute is dynamically set then, disabled behavior is working partially.

What are the steps to reproduce?

1. Open the stakbltiz
   https://stackblitz.com/edit/angular-6rguum?file=app/button-toggle-exclusive-example.html
2. check the checkbox
3. click on the button under button group and observe the behavior, buttons are still selectable but only first time

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

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

Angular 5+
Material 5+
TS 2.4+

Is there anything else we should know?

[angular-automatic-lock-bot]

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