Uncaught TypeError when clicking Connect button

I have created a simple testing website:

```

```

When I open this site in Chrome and click the "Connect" button, the following error appears in the console:

> Uncaught (in promise) TypeError: Failed to resolve module specifier "tslib". Relative references must start with either "/", "./", or "../".

Then the serial port chooser dialog opens, but nothing else happens. Selecting a device does nothing.