add cleanup internal transformer

lib/6to5/transformation/transformers/index.js

@@ -92,5 +92,7 @@ module.exports = {
   "minification.removeDebugger":           require("./minification/remove-debugger"),
   "minification.removeConsoleCalls":       require("./minification/remove-console-calls"),
   "minification.deadCodeElimination":      require("./minification/dead-code-elimination"),
-  "minification.renameLocalVariables":     require("./minification/rename-local-variables")
+  "minification.renameLocalVariables":     require("./minification/rename-local-variables"),
+
+  _cleanUp:                                require("./internal/cleanup")
 };

lib/6to5/transformation/transformers/internal/cleanup.js

@@ -0,0 +1,5 @@
+exports.SequenceExpression = function (node) {
+  if (node.expressions.length === 1) {
+    return node.expressions[0];
+  }
+};