Issue 3588 - Worked around fetch() API not supporting chrome-extension:// URLs on Chrome <47

lib/polyfills/fetch.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-2016 Eyeo GmbH
  *
  * Adblock Plus is free software: you can redistribute it and/or modify
  * it under the terms of the GNU General Public License version 3 as
  * published by the Free Software Foundation.
  *
  * Adblock Plus is distributed in the hope that it will be useful,
  * but WITHOUT ANY WARRANTY; without even the implied warranty of
  * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
  * GNU General Public License for more details.
  *
  * You should have received a copy of the GNU General Public License
  * along with Adblock Plus.  If not, see <http://www.gnu.org/licenses/>.
  */
 
 "use strict";
 
 (function(global)
 {
-  if ("fetch" in global)
-    return;
-
   function Response(xhr)
   {
     this._xhr = xhr;
   }
   Response.prototype = {
     get ok()
     {
       return this._xhr.status >= 200 && this._xhr.status <= 299;
     },
     text: function()
     {
       return Promise.resolve(this._xhr.responseText);
     }
   };
 
-  global.fetch = function(url)
+  function fetch(url)
   {
     return new Promise(function(resolve, reject)
     {
       var xhr = new XMLHttpRequest();
 
       xhr.onload = function()
       {
         resolve(new Response(xhr));
       };
 
       xhr.onerror = xhr.onabort = function()
       {
         reject(new TypeError("Failed to fetch"));
       };
 
       xhr.overrideMimeType("text/plain");
       xhr.open("GET", url);
       xhr.send();
     });
-  };
+  }
+
+  // While the Fetch API is natively supported since Chrome 42, before
+  // Chrome 47 it failed to fetch files from within the extension bundle.
+  // https://code.google.com/p/chromium/issues/detail?id=466876
+  var builtinFetch = global.fetch;
+  if (builtinFetch)
+    global.fetch = function(url, init)
+    {
+      return builtinFetch(url, init).catch(function(reason)
+      {
+        if (new URL(url, document.URL).protocol == "chrome-extension:")
+          return fetch(url)
+        throw reason;
+      });
+    };
+  else
+    global.fetch = fetch;
 })(this);