checkbox a11y issue: Partially done task is getting read out as 'Checkbox not checked'.

[tinayuangao]

Bug, feature request, or proposal:

Bug

What is the expected behavior?

Partially done task should read out as 'indeterminate'.

What is the current behavior?

Partially done task is getting read out as 'Checkbox not checked'. It might cause confusion to end user.

What are the steps to reproduce?

1. Click on Checkbox.2. Navigate over nested checklist.

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

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

Chromevox Chrome OS Version 61.0.3163.100 (Official Build) (64-bit) , Voiceover (mac OS version: 10.12.6)

Is there anything else we should know?

[tinayuangao]

It's the same behavior as native checkbox.

We could change it by aria-checked but I think we should make it align with native checkbox.

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