Import //fetch from Web Platform Tests.

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

+if (this.document === undefined) {
+  importScripts("/resources/testharness.js");
+  importScripts("../resources/utils.js");
+  importScripts("/common/utils.js");
+  importScripts("/common/get-host-info.sub.js");
+}
+
+/* Check preflight is ok if status is ok status (200  to 299)*/
+function corsPreflightStatus(desc, corsUrl, preflightStatus) {
+  var uuid_token = token();
+  var url = corsUrl;
+  var requestInit = {"mode": "cors"};
+  /* Force preflight */
+  requestInit["headers"] = {"x-force-preflight": ""};
+
+  var urlParameters = "?token=" + uuid_token + "&max_age=0";
+  urlParameters += "&allow_headers=x-force-preflight";
+  urlParameters += "&preflight_status=" + preflightStatus;
+
+  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");
+      if (200 <= preflightStatus && 299 >= preflightStatus) {
+        return fetch(url + urlParameters, requestInit).then(function(resp) {
+          assert_equals(resp.status, 200, "Response's status is 200");
+          assert_equals(resp.headers.get("x-did-preflight"), "1", "Preflight request has been made");
+        });
+      } else {
+        return promise_rejects(test, new TypeError(), fetch(url + urlParameters, requestInit));
+      }
+    });
+  }, desc);
+}
+
+var corsUrl = get_host_info().HTTP_REMOTE_ORIGIN + dirname(location.pathname) + RESOURCES_DIR + "preflight.py";
+for (status of [200, 201, 202, 203, 204, 205, 206,
+                300, 301, 302, 303, 304, 305, 306, 307, 308,
+                400, 401, 402, 403, 404, 405,
+                501, 502, 503, 504, 505])
+  corsPreflightStatus("Preflight answered with status " + status, corsUrl, status);
+
+done();