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

src/babel/transformation/modules/_default.js

@@ -12,10 +12,10 @@ var remapVisitor = {
     }
   },
 
-  Identifier(node, parent, scope, formatter) {
+  ReferencedIdentifier(node, parent, scope, formatter) {
     var remap = formatter.internalRemap[node.name];
 
-    if (this.isReferencedIdentifier() && remap && node !== remap) {
+    if (remap && node !== remap) {
       if (!scope.hasBinding(node.name) || scope.bindingIdentifierEquals(node.name, formatter.localImports[node.name])) {
         return remap;
       }

src/babel/transformation/modules/system.js

@@ -1,5 +1,6 @@
 import DefaultFormatter from "./_default";
 import AMDFormatter from "./amd";
+import object from  "../../helpers/object";
 import * as util from  "../../util";
 import last from "lodash/array/last";
 import each from "lodash/collection/each";
@@ -143,7 +144,7 @@ export default class SystemFormatter extends AMDFormatter {
       ]));
     }
 
-    this.internalRemap = {};
+    this.internalRemap = object();
 
     this._addImportSource(last(nodes), node);
   }

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"
+}