add warning for ignored named exports

Author: yyx990803

lib/loader.js

@@ -167,6 +167,16 @@ module.exports = function (content) {
           ? getRequireForImport('script', script, 0)
           : getRequire('script', script, 0)
       )
+    // check and warn named exports
+    if (!this.minimize) {
+      output +=
+        'if (__vue_script__ &&\n' +
+        '    __vue_script__.__esModule &&\n' +
+        '    Object.keys(__vue_script__).length > 1) {\n' +
+        '  console.warn("[vue-loader] ' + path.relative(process.cwd(), filePath) +
+          ': named exports in *.vue files are ignored.")' +
+        '}\n'
+    }
   }
 
   // add require for template