Revert of [XMLHttpRequest] Stop throwing for network error in async mode

LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js

Index: LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js
diff --git a/LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js b/LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js
index 28b9b8e91b9cf7bd6352e057ee94304a3c8a0445..6f920bcf4ac954263f4c6f04721de2c56f3a0bef 100644
--- a/LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js
+++ b/LayoutTests/http/tests/xmlhttprequest/resources/cross-origin-unsupported-url.js
@@ -20,7 +20,24 @@
     if (contentType)
         xhr.setRequestHeader('Content-Type', contentType);
 
-    shouldNotThrow('xhr.send()');
+    if (self.importScripts) {
+        // Initiating the load on the main thread is not performed synchronously,
+        // so send() will not have an exception code set by the time it
+        // completes in the Worker case. Hence, no exception will be
+        // thrown by the operation.
+        shouldNotThrow('xhr.send()');
+    } else {
+        // The implementation of send() throws an exception if an
+        // exception code has been set, regardless of the sync flag.
+        // The spec restricts this to sync only, but as error progress
+        // events provide no actionable information, it is more helpful
+        // to the user to not follow spec.
+        //
+        // As the initiation of the request happens synchronously in send(),
+        // and it is determined that it is to an unsupported CORS URL, an
+        // exception is expected to be thrown.
+        shouldThrow('xhr.send()');
+    }
 }
 
 var withContentType = true;