ServiceWorker polyfill Fetch implementation.

Source/modules/serviceworkers/polyfills/fetchPolyfill.js

+// Copyright 2014 The Chromium Authors. All rights reserved.
+// Use of this source code is governed by a BSD-style license that can be
+// found in the LICENSE file.
+
+// A simple, incomplete implementation of Fetch, intended to facilitate end
+// to end serviceworker testing.
+
+// See http://fetch.spec.whatwg.org/#fetch-method
+
+(function (global) {
+    var _castToRequest = function (item) {

[falken, 2014/06/06 17:29:03]

No space after "function"

+        if (typeof item === 'string') {
+            item = new Request({
+                url: item
+            });
+        }
+        return item;
+    };
+
+    // FIXME: Support init argument to fetch.
+    var fetch = function(request) {
+        request = _castToRequest(request);
+
+        return new Promise(function(resolve, reject) {
+            // FIXME: Use extra headers from |request|.
+            var xhr = new XMLHttpRequest();
+            xhr.open(request.method, request.url, true /* async */);
+            xhr.send(null);
+            xhr.onreadystatechange = function() {
+                if (xhr.readyState !== 4) return;
+
+                var response = new Response({

[falken, 2014/06/06 17:29:03]

The Response ctor changed in https://codereview.chromium.org/304233017/

Now the first argument is a Blob for the body, and the second is the
ResponseInit dictionary. You can just pass null for the body to mean no body.

[horo, 2014/06/09 05:25:12]

I noticed that passing a blob created by XMLHttpRequest to ServiceWorker's
respondWith() causes DCHECK failure.
 https://crbug.com/382311
And I created a patch to fix this.
 https://codereview.chromium.org/315863003/

I think after this patch is landed we can pass xhr.response to Response like
xhrBlobFetchPromise() in this.
https://code.google.com/p/chromium/issues/attachmentText?id=382311&aid=382311...

+                    status: xhr.status,
+                    statusText: xhr.statusText,
+                    // FIXME: Set response.headers.
+                    // FIXME: Set response.method when available.
+                    // FIXME: Set response.url when available.
+                    // FIXME: Set response.body when available.
+                });
+
+                if (xhr.status === 200) {
+                    resolve(response);
+                } else {
+                    reject(response);
+                }
+            };
+        });
+    };
+
+    global.fetch = global.fetch || fetch;
+}(self));  // window or worker global scope