Import //fetch from Web Platform Tests.

third_party/WebKit/LayoutTests/external/wpt/fetch/api/cors/cors-preflight-redirect.js

+if (this.document === undefined) {
+  importScripts("/resources/testharness.js");
+  importScripts("../resources/utils.js");
+  importScripts("/common/utils.js");
+  importScripts("/common/get-host-info.sub.js");
+}
+
+function corsPreflightRedirect(desc, redirectUrl, redirectLocation, redirectStatus, redirectPreflight) {
+  var uuid_token = token();
+  var url = redirectUrl;
+  var urlParameters = "?token=" + uuid_token + "&max_age=0";
+  urlParameters += "&redirect_status=" + redirectStatus;
+  urlParameters += "&location=" + encodeURIComponent(redirectLocation);
+
+  if (redirectPreflight)
+    urlParameters += "&redirect_preflight";
+  var requestInit = {"mode": "cors", "redirect": "follow"};
+
+  /* Force preflight */
+  requestInit["headers"] = {"x-force-preflight": ""};
+  urlParameters += "&allow_headers=x-force-preflight";
+
+  promise_test(function(test) {
+    fetch(RESOURCES_DIR + "clean-stash.py?token=" + uuid_token).then(function(resp) {
+      assert_equals(resp.status, 200, "Clean stash response's status is 200");
+      return promise_rejects(test, new TypeError(), fetch(url + urlParameters, requestInit));
+    });
+  }, desc);
+}
+
+var redirectUrl = get_host_info().HTTP_REMOTE_ORIGIN + dirname(location.pathname) + RESOURCES_DIR + "redirect.py";
+var locationUrl =  get_host_info().HTTP_REMOTE_ORIGIN + dirname(location.pathname) + RESOURCES_DIR + "preflight.py";
+
+for (var code of [301, 302, 303, 307, 308]) {
+  /* preflight should not follow the redirection */
+  corsPreflightRedirect("Redirection " + code + " on preflight failed", redirectUrl, locationUrl, code, true);
+  /* preflight is done before redirection: preflight force redirect to error */
+  corsPreflightRedirect("Redirection " + code + " after preflight failed", redirectUrl, locationUrl, code, false);
+}
+
+done();