Update javascript_natives/index.js script

This change also includes a manual run of the script to update
NativeFunctions.js.

Change-Id: I9ce1c4979674d9513cb7259e3e311e30b13de9ed
Bug: 1255619
Doc: https://goo.gle/devtools-js-argument-hints
Reviewed-on: https://chromium-review.googlesource.com/c/devtools/devtools-frontend/+/3399837
Reviewed-by: Mathias Bynens <mathias@chromium.org>
Commit-Queue: Johan Bay <jobay@chromium.org>
diff --git a/scripts/javascript_natives/test.d.ts b/scripts/javascript_natives/test.d.ts
new file mode 100644
index 0000000..8caa11e
--- /dev/null
+++ b/scripts/javascript_natives/test.d.ts
@@ -0,0 +1,9 @@
+interface Array<T> {
+  at(index: number): T|undefined;
+  diffSig(oneSig: number): T|undefined;
+}
+
+interface ReadonlyArray<T> {
+  at(index: number): T|undefined;
+  diffSig(twoSig: number): T|undefined;
+}