Issue 6292 - Make issue reporter compatible with test server

ext/content.js

 /*
  * This file is part of Adblock Plus <https://adblockplus.org/>,
  * Copyright (C) 2006-present 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
@@ skipping 84 matching lines @@
       });
     }
   };
 
   if (!("tabs" in browser))
     browser.tabs = new Map([[0, {url: "example.com"}]]);
 
   browser.tabs.get = (...args) =>
   {
     // Extend browser.tabs.get()
-    let result = Map.prototype.get.apply(browser.tabs, args);
-    const error = new Error("Tab cannot be found");

[a.giammarchi, 2018/03/12 15:53:08]

Nit: when there is a result, there's no actually need to create a new Error,
involve the stack, etc.
Also it's quite weird to find a let followed by const for two variables that
won't change in scope.

This is how I would've written this:

```
const result = Map.prototype.get.apply(browser.tabs, args);
const error = result ? null : new Error("Tab cannot be found");
return result ? Promise.resolve(result) : Promise.reject(error);
```

`null` is always very cheap (no scope resolution like `undefined` might have)
and it should never de-opt from engine optimizations.

[saroyanm, 2018/03/12 16:09:52]

Done. I'm using one ternary operator to make this simpler.

-    return result ? Promise.resolve(result) : Promise.reject(error);
+    const result = Map.prototype.get.apply(browser.tabs, args);
+    return (result ? Promise.resolve(result) :
+      Promise.reject(new Error("Tab cannot be found")));
   };
 }());