fix back button, add deprecation warnings

ionic/components/menu/menu-toggle.ts

@@ -34,6 +34,11 @@ export class MenuToggle extends Ion {
     this.app = app;
     this.viewCtrl = viewCtrl;
     this.withinNavbar = !!navbar;
+
+    // Deprecation warning
+    if (this.withinNavbar && elementRef.nativeElement.tagName === 'A') {
+      console.warn('Menu toggles within a navbar should use <button menu-toggle> instead of <a toggle>')
+    }
   }
 
   /**

ionic/components/menu/test/multiple/page1.html

@@ -1,8 +1,8 @@
 
 <ion-navbar *navbar>
-  <a [menu-toggle]="activeMenu">
+  <button [menu-toggle]="activeMenu">
     <icon menu></icon>
-  </a>
+  </button>
   <ion-title>
     Multiple Menus
   </ion-title>