Fix null aware generic invocation.

test/codegen/language/nullaware_opt_test.dart

 // Copyright (c) 2015, 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.
 //
 // VMOptions=--optimization_counter_threshold=5
 //
 // Basic null-aware operator test that invokes the optimizing compiler.
 
 import "package:expect/expect.dart";
 
@@ skipping 14 matching lines @@
   var c;
   var d = new C(5);
   Expect.equals(null, c?.m(bomb()));
   Expect.equals(null, getNull()?.anything(bomb()));
   Expect.equals(1, d?.m(1));
 
   Expect.equals(1, new C(1)?.f);
   Expect.equals(null, c?.v);
   Expect.equals(10, c ?? 10);
   Expect.equals(d, d ?? bomb());
+  Expect.equals(3, [[3]]?.expand((i) => i).toList()[0]);
+  Expect.equals(null, (null as List<List<int>>)?.expand((i) => i)?.toList());
 
   var e;
   // The assignment to e is not executed since d != null.
   d ??= e ??= new C(100);
   Expect.equals(null, e);
   e ??= new C(100);
   Expect.equals(100, e?.f);
   e?.f ??= 200;
   Expect.equals(100, e?.f);
 
@@ skipping 16 matching lines @@
   c?.v;
   c?.m(bomb());
 }
 
 main() {
   for (int i = 0; i < 10; i++) {
     test();
     test2();
   }
 }