Issue 6292 - Make issue reporter compatible with test server

ext/content.js

Index: ext/content.js
===================================================================
--- a/ext/content.js
+++ b/ext/content.js
@@ -67,7 +67,6 @@
   browser.runtime.sendMessage = (message, responseCallback) =>
   {
     let messageId = ++maxMessageId;
-
     ext.backgroundPage._sendRawMessage({
       type: "message",
       messageId,
@@ -96,4 +95,15 @@
       });
     }
   };
+
+  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);
+  };
 }());