a.js

/*---
description: >
  Error thrown when invoking the instance's `then` method (rejecting Promise)
esid: sec-promise.allSettled
info: |
  6. Let result be PerformPromiseAllSettled(iteratorRecord, C, promiseCapability).
  7. If result is an abrupt completion, then
    a. If iteratorRecord.[[Done]] is false, set result to IteratorClose(iteratorRecord, result).
    b. IfAbruptRejectPromise(result, promiseCapability).

  Runtime Semantics: PerformPromiseAllSettled

  z. Perform ? Invoke(nextPromise, "then", « resolveElement, rejectElement »).
flags: [async]
---*/

var promise = new Promise(function() {});
var error = new Test262Error();

promise.then = function() {
  throw error;
};

Promise.allSettled([promise]).then(function() {
  throw new Test262Error('The promise should be rejected');
}, function(reason) {
  assert.sameValue(reason, error);
}).then($DONE, $DONE);

Using shim.js

var assert = require('assert');
var allSettled = require('promise.allsettled');
 
allSettled.shim(); // will be a no-op if not needed
 
var promise = new Promise(function() {});
var error = new Error();

promise.then = function() {
  throw error;
};

function $DONE(arg) {
  assert.strictEqual(arg, undefined);
  console.log('ok');
}

Promise.all([promise]).then(function() {
  throw 'The promise should be rejected';
}, function(reason) {
  assert.strictEqual(reason, error);
}).then($DONE, $DONE);

Promise.allSettled([promise]) // shim is throwing here and not rejecting the promise per spec text
  .then(function() {
    throw 'The promise should be rejected';
  }, function(reason) {
    assert.strictEqual(reason, error); // this is the current spec text today
  }).then($DONE, $DONE);

Promise.allSettled([promise])
  .then(function(arr) {
    arr[0] // sounds like it should be { status: 'rejected', reason: error }
  });