Allow private properties in named exports (fix: vuejs#759)

Author: matt-oconnell

lib/loader.js

@@ -389,7 +389,7 @@ module.exports = function (content) {
     // check named exports
     output +=
       'if (Component.esModule && Object.keys(Component.esModule).some(function (key) {' +
-        'return key !== "default" && key !== "__esModule"' +
+        'return key !== "default" && key.substr(0, 2) !== "__"' +
       '})) {' +
         'console.error("named exports are not supported in *.vue files.")' +
       '}\n'