fix microtask implementation in engines with MutationObserver, but …

…without `document`, #865, #866

CHANGELOG.md

@@ -2,6 +2,7 @@
 ##### Unreleased
 - `String#replaceAll` moved to the stable ES, [per Jun TC39 meeting](https://github.com/tc39/notes/blob/master/meetings/2020-06/june-2.md#stringprototypereplaceall-for-stage-4)
 - `process.nextTick` have a less priority when `Promise` in the microtask implementation, [#855](https://github.com/zloirock/core-js/issues/855)
+- Fixed microtask implementation in engines with `MutationObserver`, but without `document`, [#865](https://github.com/zloirock/core-js/issues/865), [#866](https://github.com/zloirock/core-js/issues/866)
 - Added a workaround for 3rd party `Reflect.set` polyfill bug, [#847](https://github.com/zloirock/core-js/issues/847)
 
 ##### 3.6.5 - 2020.04.09

packages/core-js/internals/microtask.js

@@ -5,6 +5,7 @@ var macrotask = require('../internals/task').set;
 var IS_IOS = require('../internals/engine-is-ios');
 
 var MutationObserver = global.MutationObserver || global.WebKitMutationObserver;
+var document = global.document;
 var process = global.process;
 var Promise = global.Promise;
 var IS_NODE = classof(process) == 'process';
@@ -34,7 +35,7 @@ if (!queueMicrotask) {
   };
 
   // browsers with MutationObserver, except iOS - https://github.com/zloirock/core-js/issues/339
-  if (MutationObserver && !IS_IOS && !IS_NODE) {
+  if (!IS_IOS && !IS_NODE && MutationObserver && document) {
     toggle = true;
     node = document.createTextNode('');
     new MutationObserver(flush).observe(node, { characterData: true });