bring back the logger build

build/build.js

@@ -37,6 +37,16 @@ rollup.rollup({
   }).code
   return write('dist/vuex.min.js', minified)
 })
+.then(function () {
+  return rollup.rollup({
+    entry: 'src/plugins/logger.js',
+    plugins: [babel()]
+  }).then(function (bundle) {
+    return write('logger.js', bundle.generate({
+      format: 'cjs'
+    }).code)
+  })
+})
 .catch(logError)
 
 function write (dest, code) {