ngShow and ngHide on directives that use comment placeholders will error

[jelbourn]

To reproduce:
Click the "Toggle" button
http://codepen.io/jelbourn/pen/mEzrOP?editors=1010#0

The problem:
Angular sometimes uses comment elements as placeholders so that a real element doesn't have to be rendered. This means the element given to a link function will be of type Comment.

Material's decorating of ngShow and ngHide uses getComputedStyle on the element, which fails because it is a comment.

cc @topherfangio as the last one to touch these lines

[ThomasBurleson]

@crisbeto - another blocker to be fixed asap