Add a test to catch missing demo messages

PR #2330 introduced a new message without a definition for it.  This
test would have caught the mistake in advance.

Change-Id: I1a7b0fa2483b791b6b16405e55af84a49d100c83

karma.conf.js

@@ -119,6 +119,8 @@ module.exports = (config) => {
       {pattern: 'test/test/assets/*', included: false},
       {pattern: 'dist/shaka-player.ui.js', included: false},
       {pattern: 'dist/locales.js', included: false},
+      {pattern: 'demo/locales/en.json', included: false},
+      {pattern: 'demo/locales/source.json', included: false},
       {pattern: 'node_modules/**/*.js', included: false},
     ],
 

test/demo/demo_unit.js

@@ -32,6 +32,18 @@ describe('Demo', () => {
     await shakaDemoMain.cleanup();
   });
 
+  it('has all messages defined', async () => {
+    const englishResponse = await fetch('/base/demo/locales/en.json');
+    const englishMessages = await englishResponse.json();
+    const sourceResponse = await fetch('/base/demo/locales/source.json');
+    const sourceMessages = await sourceResponse.json();
+
+    for (const id of Object.values(shakaDemo.MessageIds)) {
+      expect(englishMessages[id]).withContext(`id=${id}, English`).toBeTruthy();
+      expect(sourceMessages[id]).withContext(`id=${id}, source`).toBeTruthy();
+    }
+  });
+
   describe('config', () => {
     it('does not have entries for invalid config options', () => {
       // We determine whether a config option has been made or not by looking at