[Checkbox] md-checkbox not clickable in next version (> v1.0.0-pre.15)

[pgbross]

In <md-checkbox> the render method has changed in PR #4726 "fix(textfield): add form validity support]". The PR moves the <input> element to be rendered before the <svg> but this prevents clicks on the svg element from working (only tested in Chrome).

Can't demo this in the playground as this is a future release at the moment.

A possible fix might be to set the z-index: 1 on the <input> element so that it is not masked by the <svg> element.