fix: do not use MutationObserver in IE11

MutationObserver is unreliable in IE11. fix #6466

Author: yyx990803

src/core/util/env.js

@@ -99,13 +99,13 @@ export const nextTick = (function () {
       // "force" the microtask queue to be flushed by adding an empty timer.
       if (isIOS) setTimeout(noop)
     }
-  } else if (typeof MutationObserver !== 'undefined' && (
+  } else if (!isIE && typeof MutationObserver !== 'undefined' && (
     isNative(MutationObserver) ||
     // PhantomJS and iOS 7.x
     MutationObserver.toString() === '[object MutationObserverConstructor]'
   )) {
     // use MutationObserver where native Promise is not available,
-    // e.g. PhantomJS IE11, iOS7, Android 4.4
+    // e.g. PhantomJS, iOS7, Android 4.4
     var counter = 1
     var observer = new MutationObserver(nextTickHandler)
     var textNode = document.createTextNode(String(counter))