ensure that codemirror/src is processed by babel

config/webpack.common.js

@@ -77,6 +77,7 @@ module.exports = (options) => {
           exclude: {
             test:    helpers.root('node_modules'),
             exclude: [  // include as a result
+              helpers.root('node_modules/codemirror/src'),
               helpers.root('node_modules/string-width'),
               helpers.root('node_modules/is-fullwidth-code-point'), // depends from string-width
             ]