tabindex on MatDatepickerToggle

[mattmcardle]

Bug, feature request, or proposal:

Feature Request

What is the expected behavior?

When specifying a tabindex of -1 on the datepicker toggle, and focus is on the datepickers associated input element,when tab is pressed the focus should skip the datepicker toggle and focus on the next focusable element. (As per https://developer.mozilla.org/en-US/docs/Web/HTML/Global_attributes/tabindex)

What is the current behavior?

When specifying a tabindex of -1 on the datepicker toggle, and focus is on the datepickers associated input element, when tab is pressed the focus goes to the datepicker toggle, ignoring the tabindex of -1.

What are the steps to reproduce?

https://stackblitz.com/edit/angular-material2-issue-xzdzcf

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

Reducing keypresses required from user to move through the form

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

Latest stable - tested in chrome.

Is there anything else we should know?

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