bug(focus indicators): Move mixin from material-experimental to material.

Author: zelliott

src/material-experimental/mdc-helpers/_mdc-helpers.scss

@@ -309,13 +309,3 @@ $mat-typography-level-mappings: (
       $theme-or-color);
   }
 }
-
-// Mixin that applies base styles required for focus indicators to render appropriately.
-// Private mixin included within `_mat-mdc-core`.
-@mixin _mat-mdc-strong-focus-indicators-base() {
-  // Ensure that focus indicator host elements are positioned so that the focus indicator
-  // pseudo element within is positioned relatively to the host.
-  .mat-mdc-focus-indicator {
-    position: relative;
-  }
-}

src/material/core/_core.scss

@@ -58,7 +58,17 @@
   }
 }
 
-// Mixin that renders all of the core MDC styles.
+// Mixin that renders all of the core MDC styles. Private mixin included within `mat-core`.
 @mixin _mat-mdc-core() {
   @include _mat-mdc-strong-focus-indicators-base();
 }
+
+// Mixin that applies base styles required for focus indicators to render appropriately.
+// Private mixin included within `_mat-mdc-core`.
+@mixin _mat-mdc-strong-focus-indicators-base() {
+  // Ensure that focus indicator host elements are positioned so that the focus indicator
+  // pseudo element within is positioned relatively to the host.
+  .mat-mdc-focus-indicator {
+    position: relative;
+  }
+}