add a has method to mirror for checking whether an id is in the map

src/utils.ts

@@ -21,7 +21,7 @@ export const mirror: Mirror = {
     return n.__sn && n.__sn.id;
   },
   getNode(id) {
-    return mirror.map[id];
+    return mirror.map[id] || null;
   },
   // TODO: use a weakmap to get rid of manually memory management
   removeNodeFromMap(n) {
@@ -33,6 +33,9 @@ export const mirror: Mirror = {
       );
     }
   },
+  has(id) {
+    return mirror.map.hasOwnProperty(id);
+  },
 };
 
 // copy from underscore and modified