Core: Support non-browser environments

Fixes gh-2133
Fixes gh-2501
Closes gh-2504
Refs gh-1950
Refs gh-1949
Refs gh-2397
Refs gh-1537
Refs gh-2504
Refs 842958e7ae

src/var/documentElement.js

@@ -0,0 +1,5 @@
+define([
+	"./document"
+], function( document ) {
+	return document.documentElement;
+});