make it illegal to export a __esModule property - #673

lib/6to5/transformation/modules/_default.js

@@ -212,6 +212,12 @@ DefaultFormatter.prototype.getExternalReference = function (node, nodes) {
   }
 };
 
+DefaultFormatter.prototype.checkExportIdentifier = function (node) {
+  if (t.isIdentifier(node, { name: "__esModule" })) {
+    throw this.file.errorWithNode(node, "Illegal export __esModule - this is used internally for CommonJS interop");
+  }
+};
+
 DefaultFormatter.prototype.exportSpecifier = function (specifier, node, nodes) {
   var inherits = false;
   if (node.specifiers.length === 1) inherits = node;
@@ -251,6 +257,7 @@ DefaultFormatter.prototype.buildExportsWildcard = function (objectIdentifier) {
 };
 
 DefaultFormatter.prototype.buildExportsAssignment = function (id, init) {
+  this.checkExportIdentifier(id);
   return util.template("exports-assign", {
     VALUE: init,
     KEY:   id