fix: False positive warning about MessageChannel (#522)

Author: eps1lon

src/flush-microtasks.js

@@ -21,20 +21,22 @@ try {
   // we can't use regular timers because they may still be faked
   // so we try MessageChannel+postMessage instead
   enqueueTask = callback => {
-    if (didWarnAboutMessageChannel === false) {
+    const supportsMessageChannel = typeof MessageChannel === 'function'
+    if (supportsMessageChannel) {
+      const channel = new MessageChannel()
+      channel.port1.onmessage = callback
+      channel.port2.postMessage(undefined)
+    } else if (didWarnAboutMessageChannel === false) {
       didWarnAboutMessageChannel = true
+
       // eslint-disable-next-line no-console
       console.error(
-        typeof MessageChannel !== 'undefined',
         'This browser does not have a MessageChannel implementation, ' +
           'so enqueuing tasks via await act(async () => ...) will fail. ' +
           'Please file an issue at https://github.com/facebook/react/issues ' +
           'if you encounter this warning.',
       )
     }
-    const channel = new MessageChannel()
-    channel.port1.onmessage = callback
-    channel.port2.postMessage(undefined)
   }
 }