Add tests of null-aware constructs in void contexts.

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

 library test;
 import self as self;
 import "dart:core" as core;
 
 class C extends core::Object {
   field core::int x = null;
   constructor •() → void
     : super core::Object::•()
     ;
 }
 static method f(self::C c) → void {
   dynamic x = let final self::C #t1 = c in #t1.==(null) ?{dynamic} null : #t1.x;
+  let final self::C #t2 = c in #t2.==(null) ?{dynamic} null : #t2.x;
 }
 static method main() → dynamic {}