better ClassDeclarations in expression contexts

lib/6to5/transformation/transformers/es6-classes.js

@@ -9,10 +9,15 @@ exports.ClassDeclaration = function (node, parent, file, scope) {
   }
 
   var newNode = new Class(node, file, scope, closure).run();
-  if (closure || t.isFunctionExpression(newNode)) {
-    return t.variableDeclaration("let", [
-      t.variableDeclarator(node.id, newNode)
-    ]);
+  if (closure) {
+    // declaration in an expression context...
+    // export default class Foo {}
+    scope.push({
+      kind: "var",
+      key: node.id.key,
+      id: node.id
+    });
+    return t.assignmentExpression("=", node.id, newNode);
   } else {
     return newNode;
   }