Don't use "var" to introduce the deferred_initializer.

tests/compiler/dart2js_extra/deferred_custom_loader_test.dart

+// Copyright (c) 2016, the Dart project authors.  Please see the AUTHORS file
+// for details. All rights reserved. Use of this source code is governed by a
+// BSD-style license that can be found in the LICENSE file.
+
+import 'package:async_helper/async_helper.dart';
+import 'package:expect/expect.dart';
+
+import 'deferred_custom_loader_lib.dart' deferred as def;
+
+void setup() native """
+// In d8 we don't have any way to load the content of the file, so just use
+// the preamble's loader.
+if (!self.dartDeferredLibraryLoader) {
+  self.dartDeferredLibraryLoader = function(uri, success, error) {
+    var req = new XMLHttpRequest();
+    req.addEventListener("load", function() {
+      eval(this.responseText);
+      success();
+    });
+    req.open("GET", uri);
+    req.send();
+  };
+}
+""";
+
+
+runTest() async {
+  setup();
+  await def.loadLibrary();
+  Expect.equals(499, def.foo());
+}
+
+main() {
+  asyncStart();
+  runTest().then((_) => asyncEnd());
+}