Issue 4579 - Fix ESLint errors

polyfill.js

Index: polyfill.js
===================================================================
--- a/polyfill.js
+++ b/polyfill.js
@@ -125,17 +125,17 @@
     };
 
     Object.defineProperty(object, name, descriptor);
   }
 
   function wrapRuntimeOnMessage()
   {
     let {onMessage} = browser.runtime;
-    let {addListener, removeListener, hasListener} = onMessage;
+    let {addListener, removeListener} = onMessage;
 
     onMessage.addListener = function(listener)
     {
       if (typeof listener != "function")
         throw new Error(invalidMessageListenerError);
 
       // Don't add the same listener twice or we end up with multiple wrappers.
       if (messageListeners.has(listener))
@@ -148,22 +148,23 @@
         if (wait instanceof Promise)
         {
           wait.then(sendResponse, reason =>
           {
             try
             {
               sendResponse();
             }
-            finally
+            catch (error)
             {
               // sendResponse can throw if the internal port is closed; be sure
               // to throw the original error.
-              throw reason;
             }
+
+            throw reason;
           });
         }
 
         return !!wait;
       };
 
       addListener.call(onMessage, wrapper);
       messageListeners.set(listener, wrapper);