Repair silly mistake

implementations/javascript/src/flex.ts

@@ -181,14 +181,14 @@ export class FlexSet<V> implements Set<V> {
     get(v: V): {item: V} | null {
         const vs = this._key(v);
         if (this.items.has(vs)) {
-            return {item: this.items[vs]};
+            return {item: this.items.get(vs)};
         } else {
             return null;
         }
     }
 
     add(v: V): this {
-        this.items[this._key(v)] = v;
+        this.items.set(this._key(v), v);
         return this;
     }