MdInputDirective uses required attribute only to add the placeholder asterisk

[calbot]

Bug, feature request, or proposal:

Bug/Proposal

What is the expected behavior?

Basically the problem is I can't use the required attribute with mdInput in my "reactive" code based form. This is because it automatically adds validators to the form control which causes problems when a field is added or removed through ngIf. These fields which are marked required will keep the required validator on the form control even after the field is no longer in the dom.

What is the current behavior?

required field is incompatible with reactive forms.

What are the steps to reproduce?

Providing a Plunker (or similar) is the best way to get the team to see your issue.
Plunker template: https://goo.gl/DlHd6U

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

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

Is there anything else we should know?

lib/input/input-container.html

Uses the "required" attribute only

<span class="mat-placeholder-required" *ngIf="_mdInputChild.required">*</span>
Maybe if there was another attribute it would be helpful for reactive forms users who don't want the automatic validators added by Angular framework...

<span class="mat-placeholder-required" *ngIf="_mdInputChild.required || _mdInputChild.mdRequired">*</span>

Basically I end up working around this by doing the following ...

  @ViewChildren(MdInputDirective)
  private inputDirectives: QueryList<MdInputDirective>;

Then I set the mdinput.required = true manually to trick material to show the asterisk so I don't have the issues with the automatically added validators.

[jelbourn]

Dupe of #2574

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