Index: src/wp-admin/css/admin-menu.css
===================================================================
--- src/wp-admin/css/admin-menu.css	(revision 27419)
+++ src/wp-admin/css/admin-menu.css	(working copy)
@@ -348,6 +348,21 @@
 	padding: 7px 0;
 	-webkit-transition: all .1s ease-in-out;
 	transition: all .1s ease-in-out;
+
+	/* Copy from .dashicons-before:before for back compat */
+	display: inline-block;
+	width: 20px;
+	height: 20px;
+	font-size: 20px;
+	line-height: 1;
+	font-family: "dashicons";
+	text-decoration: inherit;
+	font-weight: normal;
+	font-style: normal;
+	vertical-align: top;
+	text-align: center;
+	-webkit-font-smoothing: antialiased;
+	-moz-osx-font-smoothing: grayscale;
 }
 
 #adminmenu div.wp-menu-image:before {