better ClassDeclaration contextual transformation

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

@@ -3,7 +3,19 @@ var util     = require("../../util");
 var t        = require("../../types");
 
 exports.ClassDeclaration = function (node, parent, file, scope) {
-  return new Class(node, file, scope, false).run();
+  var closure = true;
+  if (t.isProgram(parent) || t.isBlockStatement(parent)) {
+    closure = false;
+  }
+
+  var newNode = new Class(node, file, scope, closure).run();
+  if (closure || t.isFunctionExpression(newNode)) {
+    return t.variableDeclaration("let", [
+      t.variableDeclarator(node.id, newNode)
+    ]);
+  } else {
+    return newNode;
+  }
 };
 
 exports.ClassExpression = function (node, parent, file, scope) {