Make component.$$.dirty just a dictionary

To be sure that we won't have any hidden behavior related to a prototype: https://davidwalsh.name/object-create-null

src/internal/Component.js

@@ -48,7 +48,7 @@ function make_dirty(component, key) {
 	if (!component.$$.dirty) {
 		dirty_components.push(component);
 		schedule_update();
-		component.$$.dirty = {};
+		component.$$.dirty = Object.create(null);
 	}
 	component.$$.dirty[key] = true;
 }