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

lib/polyfills/fetch.js

Index: lib/polyfills/fetch.js
===================================================================
--- a/lib/polyfills/fetch.js
+++ b/lib/polyfills/fetch.js
@@ -19,9 +19,6 @@
 
 (function(global)
 {
-  if ("fetch" in global)
-    return;
-
   function Response(xhr)
   {
     this._xhr = xhr;
@@ -37,7 +34,7 @@
     }
   };
 
-  global.fetch = function(url)
+  function fetch(url)
   {
     return new Promise(function(resolve, reject)
     {
@@ -57,5 +54,22 @@
       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);