Allow root functions defined via @plugin

test/less/plugin/plugin-root.js

@@ -0,0 +1,11 @@
+functions.addMultiple({
+    "test-detached-ruleset" : function() {
+        var rule = new tree.Rule('prop', new tree.Anonymous('value'));
+        return new tree.DetachedRuleset(new tree.Ruleset("", [ rule ]));
+    },
+    "test-directive": function() {
+        return new tree.Directive("@charset", new tree.Anonymous('"utf-8"'));
+    },
+    // Functions must return something. Must 'return true' if they produce no output.
+    "test-undefined": function() { }
+});