create null object when clearing internalRemap instead of using an object inherited plain object - fixes #1489

test/core/fixtures/transformation/misc/regression-1489/actual.js

@@ -0,0 +1,2 @@
+import toString from "foo";
+toString;

test/core/fixtures/transformation/misc/regression-1489/expected.js

@@ -0,0 +1,13 @@
+System.register(["foo"], function (_export) {
+  "use strict";
+
+  var toString;
+  return {
+    setters: [function (_foo) {
+      toString = _foo["default"];
+    }],
+    execute: function () {
+      toString;
+    }
+  };
+});

test/core/fixtures/transformation/misc/regression-1489/options.json

@@ -0,0 +1,3 @@
+{
+  "modules": "system"
+}