Fix karma config - esm crashes when .ts file is requested

Change-Id: I92e391510b913f952b35dde5178d84fc74ad3216
diff --git a/polygerrit-ui/karma.conf.js b/polygerrit-ui/karma.conf.js
index a0358f1..879a5c8 100644
--- a/polygerrit-ui/karma.conf.js
+++ b/polygerrit-ui/karma.conf.js
@@ -83,8 +83,11 @@
     esm: {
       nodeResolve: {
         // By default, it tries to use page.mjs file instead of page.js
-        // when importing 'page/page'.
-        extensions: ['.js'],
+        // when importing 'page/page', so we shouldn't use .mjs extension
+        // in node resolve.
+        // The .ts extension is required to display source code in browser
+        // (otherwise esm plugin crashes)
+        extensions: ['.js', '.ts'],
       },
       moduleDirs: getModulesDir(),
       // Bazel and yarn uses symlinks for files.