The .mjs test runs through vm.SourceTextModule's importModuleDynamically
callback. Whether that host callback executes synchronously during the
import() expression or is queued as a microtask is V8-version-dependent.
On Node 22+ V8 it runs synchronously, so the chunk's webpackChunk.push
lands before the next-line script.onload(). On Node 10/12/14/16/18/20 V8
the callback is deferred — script.onload() then runs first, sees
installedChunks[id] still as [resolve, reject, promise], and the JSONP
runtime rejects with "Loading chunk chunk1 failed".

await the import(webpackIgnore) so the chunk install is observed before
script.onload() regardless of V8 version, and flatten the nested then()
chain into async/await.

The trailing `import("./chunk1.css")` / `import("./chunk2.css")` in
prefetch-preload-module-jsonp and prefetch-preload each install a
stylesheet link and a <script> for the JS half. The harness never
invokes those scripts' onload, so the JSONP runtime sits on a
setTimeout(onScriptComplete, chunkLoadTimeout=120000ms). On Node 10
the integration suite runs longer than 120s, the timer fires while a
later test is executing, loadingEnded builds

  ChunkLoadError: Loading chunk 945 failed.
  (timeout: https://example.com/public/path/chunk1-css.js)

and the unhandled rejection lands inside the unrelated test
(TestCasesProdGlobalUsed > coffee-loader > should compile). Newer Node
finishes the suite before 120s elapses, which is why this only shows
up on old Node. Attach a no-op catch so the eventual rejection is
observed and dropped.