use ES6 Map if we can (#602)

src/Explorer/Tree/ResourceTreeAdapter.tsx

@@ -930,7 +930,7 @@ export class ResourceTreeAdapter implements ReactAdapter {
   }
 
   private cleanupDatabasesKoSubs(): void {
-    this.koSubsDatabaseIdMap.keys().forEach((databaseId: string) => {
+    for (const databaseId of this.koSubsDatabaseIdMap.keys()) {
       this.koSubsDatabaseIdMap.get(databaseId).forEach((sub: ko.Subscription) => sub.dispose());
       this.koSubsDatabaseIdMap.delete(databaseId);
 
@@ -939,7 +939,7 @@ export class ResourceTreeAdapter implements ReactAdapter {
           .get(databaseId)
           .forEach((collectionId: string) => this.cleanupKoSubsForCollection(databaseId, collectionId));
       }
-    });
+    }
   }
 
   private cleanupCollectionsKoSubs(databaseId: string, existingCollectionIds: string[]): void {