Make webpack fail for missing exports

config/webpack.config.js

@@ -141,6 +141,7 @@ var config = {
     ],
 
     noParse: [/monaco-editor\/\w+\/vs\//],
+    strictExportPresence: true,
   },
 
   plugins: [

spec/javascripts/monitoring/mock_data.js

 /* eslint-disable quote-props, indent, comma-dangle */
 
-const metricsGroupsAPIResponse = {
+export const mockApiEndpoint = `${gl.TEST_HOST}/monitoring/mock`;
+
+export const metricsGroupsAPIResponse = {
   'success': true,
   'data': [
     {