Add tests of null-aware constructs in void contexts.

pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect

Index: pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect
diff --git a/pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect b/pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect
index 48478c83fab5c40685d5543e1cd1d1a57acefbff..3d84f9ca84a48614372e37878635b574724d8f4b 100644
--- a/pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect
+++ b/pkg/front_end/testcases/inference/null_aware_property_get.dart.strong.expect
@@ -10,5 +10,6 @@ class C extends core::Object {
 }
 static method f(self::C c) → void {
   core::int x = let final self::C #t1 = c in #t1.==(null) ?{core::int} null : #t1.{self::C::x};
+  let final self::C #t2 = c in #t2.==(null) ?{core::int} null : #t2.{self::C::x};
 }
 static method main() → dynamic {}