Extending HTMLElement crashes in iOS 9

[davidaq]

Describe the bug
A web app fully developed in svelte (without SSR) displays blank because the framework fails to initialize. With some effort of debugging, I found out the cause was here in svelte/internal:

let SvelteElement;
if (typeof HTMLElement !== 'undefined') {
    SvelteElement = class extends HTMLElement { // culprit

Interestingly HTMLElement is not undefined in iOS 9 but its not a class (function) either.
typeof HTMLElement equals to "object" in iOS 9.This causes the script to throw and stop running before anything can be seen.

The error wasn't that apparent actually. I used Babel to transpile es6 to es5, which changes class to functions. After transpile there is a native function check:

function _isNativeFunction(fn) {
  return Function.toString.call(fn).indexOf("[native code]") !== -1;
}

You can try this out in Babel REPL by writing class A extends HTMLElement {}. And with this, the browser would run Function.toString.call(HTMLElement), which is also faulty if HTMLElement is not a function.

Logs
The log on iOS 9 is quite vague:

TypeError: Type error

To Reproduce
Just run something written with Svelte on a iOS 9 device. If you can't find one, you can try it out on XCode iPhone simulator just as I did.

Expected behavior
Forget about SvelteElement if HTMLElement isn't extendable.

Information about your Svelte project:

• Your browser and the version: Safari and iOS UIWebView

• Your operating system: (e.x. OS X 10, Ubuntu Linux 19.10, Windows XP, etc)

• Svelte version: 3.12.1

• Whether your project uses Webpack or Rollup: both, the same

Severity
Severe as iOS 9 users are completely unusable because of SvelteElement which is not used at all. There's still 0.7% of my app users with this OS version.

Additional context
I'm actually curious why SvelteElement wasn't dropped by rollup. It's not used at all in my app.

[Conduitry]

This not getting treeshaken by Rollup is something I actually opened an issue for (rollup/rollup#3115) and which was fixed in 1.21.4. Upgrading Rollup should resolve this (by having this code not even be present in the bundle), but changing this to typeof HTMLElement === 'function' also might make sense.

[davidaq]

@Conduitry Wow nice to see a work around already. Having tree shaking working properly solves my case.