Fix deferred bug: when type argument is never instantiated

tests/compiler/dart2js_extra/deferred/uninstantiated_type_variable_test.dart

Index: tests/compiler/dart2js_extra/deferred/uninstantiated_type_variable_test.dart
diff --git a/tests/compiler/dart2js_extra/deferred/uninstantiated_type_variable_test.dart b/tests/compiler/dart2js_extra/deferred/uninstantiated_type_variable_test.dart
new file mode 100644
index 0000000000000000000000000000000000000000..17f60458064514341605ab6c04a722a24fbc57a8
--- /dev/null
+++ b/tests/compiler/dart2js_extra/deferred/uninstantiated_type_variable_test.dart
@@ -0,0 +1,20 @@
+// Copyright (c) 2017, 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:expect/expect.dart';
+import 'uninstantiated_type_variable_lib.dart' deferred as lib;
+
+/// Regression test: if a type variable is used, but not instantiated, it still
+/// needs to be mapped to the deferred unit where it is used.
+///
+/// If not, we may include it in the main unit and may not see that the base
+/// class is not added to the main unit.
+main() {
+  lib.loadLibrary().then((_) {
+    dontInline(new lib.C()).box.value;
+  });
+}
+
+@NoInline()
+dontInline(x) => x;