Enable the downlevelIteration TypeScript compiler option

The TypeScript target is ECMAScript 2017, and Babel performs all the
actual compilation, so this option has no effect except at the type
level.
diff --git a/tsconfig.json b/tsconfig.json
index f37c6c5..acc09ae 100644
--- a/tsconfig.json
+++ b/tsconfig.json
@@ -2,6 +2,7 @@
   "compilerOptions": {
     "allowSyntheticDefaultImports": true,
     "baseUrl": ".",
+    "downlevelIteration": true,
     "isolatedModules": true,
     "lib": [
       "dom",