add illegal top level this transformer - fixes #562

lib/6to5/transformation/transform.js

@@ -52,6 +52,7 @@ _.each({
   "spec.noForInOfAssignment":           require("./transformers/spec/no-for-in-of-assignment"),
   "spec.setters":                       require("./transformers/spec/setters"),
   "spec.blockScopedFunctions":          require("./transformers/spec/block-scoped-functions"),
+  "spec.illegalTopLevelThis":           require("./transformers/spec/illegal-top-level-this"),
 
   "playground.malletOperator":          require("./transformers/playground/mallet-operator"),
   "playground.methodBinding":           require("./transformers/playground/method-binding"),

lib/6to5/transformation/transformers/spec/illegal-top-level-this.js

@@ -0,0 +1,8 @@
+exports.FunctionDeclaration =
+exports.FunctionExpression = function (node, parent, scope, context) {
+  context.skip();
+};
+
+exports.ThisExpression = function (node, parent, scope, context, file) {
+  throw file.errorWithNode(node, "Top level `this` is not allowed", ReferenceError);
+};