fix(runtime): don't overwrite native Event/EventTarget on web

[nachooya]

Summary

@react-native-harness/runtime's initialize.ts unconditionally assigns event-target-shim's classes to globalThis.Event and globalThis.EventTarget. That polyfill is required on RN's JavaScriptCore runtime (which has no DOM), but on web (RN Web running in a real browser) it clobbers the browser's native ctors.

Safari's native EventTarget.dispatchEvent() does an internal brand check on its argument and throws

TypeError: Argument 1 ('event') to EventTarget.dispatchEvent must be an instance of Event

when a polyfill instance is dispatched onto a native EventTarget — even though the polyfill behaves mostly like Event. That's enough to break any DOM-event-driven flow in the page.

Fix

Only install the polyfill when a native ctor is not already present:

if (typeof globalThis.Event !== 'function') {
  globalThis.Event = Shim.Event;
}
if (typeof globalThis.EventTarget !== 'function') {
  globalThis.EventTarget = Shim.EventTarget;
}

• On RN/JSC: typeof globalThis.Event !== 'function', polyfill installs — unchanged behaviour.
• On RN Web / any browser: Event is a native ctor, polyfill is skipped, native dispatch keeps working.

Test plan

• pnpm nx run-many -t build typecheck lint -p @react-native-harness/runtime — green.

[vercel]

@nachooya is attempting to deploy a commit to the Callstack Team on Vercel.

A member of the Team first needs to authorize it.