ref: upgrade to ES2017

tsconfig.json

@@ -2,7 +2,7 @@
   "compilerOptions": {
     "module": "NodeNext",
     "moduleResolution": "NodeNext",
-    "target": "ES2016",
+    "target": "ES2017",
     "outDir": "dist/out",
     "lib": [
       "ES5",