fix(material/slide-toggle): Allow typography mixin to consume 2018 style configs

See angular#21059 for context.

Author: mmalerba

src/material/slide-toggle/_slide-toggle-theme.scss

@@ -1,6 +1,7 @@
 @import '../core/style/private';
 @import '../core/theming/palette';
 @import '../core/theming/theming';
+@import '../core/typography/typography';
 @import '../core/typography/typography-utils';
 
 @mixin _mat-slide-toggle-checked($palette, $thumb-checked-hue) {
@@ -73,6 +74,7 @@
 }
 
 @mixin mat-slide-toggle-typography($config-or-theme) {
+  $config: mat-private-typography-normalized-config(mat-get-typography-config($config-or-theme));
   $config: mat-get-typography-config($config-or-theme);
   .mat-slide-toggle-content {
     font-family: mat-font-family($config);