Avoid new Set([iterable]) for thenables

Fixes facebook#14583

Using `new Set([iterable])` does not work with IE11's non-compliant Set
implementation. By avoiding this pattern we don't need to require a Set
polyfill for IE11

packages/react-reconciler/src/ReactFiberUnwindWork.js

@@ -214,7 +214,8 @@ function throwException(
         // attach another listener to flip the boundary back to its normal state.
         const thenables: Set<Thenable> = (workInProgress.updateQueue: any);
         if (thenables === null) {
-          workInProgress.updateQueue = (new Set([thenable]): any);
+          workInProgress.updateQueue = (new Set(): any);
+          workInProgress.updateQueue.add(thenable);
         } else {
           thenables.add(thenable);
         }