demo(slide-toggle): fix slide-toggle required demo (#3793)

Fixes the slide-toggle required demo, where the component will be compared with the native required state.

Basically in the demo, the form should be validated native and should show the browsers popup if the slide-toggle is required.

src/demo-app/slide-toggle/slide-toggle-demo.html

@@ -14,7 +14,7 @@
 
   <p>Example where the slide toggle is required inside of a form.</p>
 
-  <form #form="ngForm" (ngSubmit)="onFormSubmit()">
+  <form #form="ngForm" (ngSubmit)="onFormSubmit()" ngNativeValidate>
 
     <md-slide-toggle name="slideToggle" required ngModel>
       Slide Toggle