Make `createTemplate` pure

Extracting `options.platform` makes it easier to test without having to
stub `process.platform`.

app/menu.js

@@ -1,9 +1,17 @@
+const isString = require('lodash/isString');
+
+
 exports.createTemplate = (options, messages) => {
+  if (!isString(options.platform)) {
+    throw new TypeError('`options.platform` must be a string');
+  }
+
   const {
     openForums,
     openNewBugForm,
     openReleaseNotes,
     openSupportPage,
+    platform,
     setupAsNewDevice,
     setupAsStandalone,
     setupWithImport,
@@ -147,7 +155,7 @@ exports.createTemplate = (options, messages) => {
     });
   }
 
-  if (process.platform === 'darwin') {
+  if (platform === 'darwin') {
     return updateForMac(template, messages, options);
   }
 

main.js

@@ -435,6 +435,7 @@ app.on('ready', () => {
 });
 
 function setupMenu(options) {
+  const { platform } = process;
   const menuOptions = Object.assign({}, options, {
     development,
     showDebugLog,
@@ -444,6 +445,7 @@ function setupMenu(options) {
     openNewBugForm,
     openSupportPage,
     openForums,
+    platform,
     setupWithImport,
     setupAsNewDevice,
     setupAsStandalone,