Add tests of null-aware constructs in void contexts.

pkg/front_end/testcases/inference/null_aware_property_get.dart

 // 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.
 
 /*@testedFeatures=inference*/
 library test;
 
 class C {
   int x;
 }
 
 void f(C c) {
   var /*@type=int*/ x = c?. /*@target=C::x*/ x;
+  c?. /*@target=C::x*/ x;
 }
 
 main() {}