Import //fetch from Web Platform Tests.

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

+if (this.document === undefined) {
+  importScripts("/resources/testharness.js");
+  importScripts("../resources/utils.js");
+  importScripts("/common/get-host-info.sub.js");
+}
+
+const url = get_host_info().HTTP_REMOTE_ORIGIN + dirname(location.pathname) + RESOURCES_DIR + "preflight.py",
+      origin = location.origin // assuming an ASCII origin
+
+function preflightTest(succeeds, withCredentials, allowMethod, allowHeader, useMethod, useHeader) {
+  return promise_test(t => {
+    let testURL = url + "?",
+        requestInit = {}
+    if (withCredentials) {
+      testURL += "origin=" + origin + "&"
+      testURL += "credentials&"
+      requestInit.credentials = "include"
+    }
+    if (useMethod) {
+      requestInit.method = useMethod
+    }
+    if (useHeader) {
+      requestInit.headers = [useHeader]
+    }
+    testURL += "allow_methods=" + allowMethod + "&"
+    testURL += "allow_headers=" + allowHeader + "&"
+
+    if (succeeds) {
+      return fetch(testURL, requestInit).then(resp => {
+        assert_equals(resp.headers.get("x-origin"), origin)
+      })
+    } else {
+      return promise_rejects(t, new TypeError(), fetch(testURL, requestInit))
+    }
+  }, "CORS that " + (succeeds ? "succeeds" : "fails") + " with credentials: " + withCredentials + "; method: " + useMethod + " (allowed: " + allowMethod + "); header: " + useHeader + " (allowed: " + allowHeader + ")")
+}
+
+preflightTest(true, false, "get", "x-test", "GET", ["X-Test", "1"])
+preflightTest(true, false, "*", "x-test", "SUPER", ["X-Test", "1"])
+preflightTest(true, false, "*", "*", "OK", ["X-Test", "1"])
+preflightTest(false, true, "*", "*", "OK", ["X-Test", "1"])
+preflightTest(false, true, "*", "", "PUT", undefined)
+preflightTest(false, true, "put", "*", "PUT", undefined)
+preflightTest(false, true, "get", "*", "GET", ["X-Test", "1"])
+preflightTest(false, true, "*", "*", "GET", ["X-Test", "1"])
+
+done();