more robust named exports check (fix for loose mode)

Author: yyx990803

lib/loader.js

@@ -30,6 +30,10 @@ var defaultLang = {
   script: 'js'
 }
 
+var checkNamedExports =
+  'if (Object.keys(__vue_exports__).some(function (key) { return key !== "default" && key !== "__esModule" })) {' +
+  'console.error("named exports are not supported in *.vue files.")}\n'
+
 module.exports = function (content) {
   this.cacheable()
   var isServer = this.options.target === 'node'
@@ -164,7 +168,7 @@ module.exports = function (content) {
     '__vue_options__ = __vue_exports__ = __vue_exports__ || {}\n' +
     // ES6 modules interop
     'if (typeof __vue_exports__.default === "object") {\n' +
-      (isProduction ? '' : 'if (Object.keys(__vue_exports__).length > 1) console.error("[vue-loader]: named exports are not supported.")\n') +
+      (isProduction ? '' : checkNamedExports) +
       '__vue_options__ = __vue_exports__ = __vue_exports__.default\n' +
     '}\n' +
     // constructor export interop