remap head variables when a closure is required in blockScoping transformer - fixes #763

test/fixtures/transformation/es6-block-scoping-exec/closure-wrap-collision.js

@@ -0,0 +1,9 @@
+for (let i = 1; i < 3; i += 1) {
+  (function () {
+    i;
+  })();
+}
+
+assert.throws(function () {
+  i;
+}, ReferenceError);