Add tests of null-aware constructs in void contexts.

Fasta desugares some expressions differently in void contexts, so to
be on the safe side, we should test that null-aware invocations and
property accesses work in both void and non-void contexts.

R=sigmund@google.com

Review-Url: https://codereview.chromium.org/2948843002 .

pkg/front_end/testcases/inference/null_aware_method_invocation.dart

@@ -11,6 +11,7 @@ class C {
 
 g(C c) {
   var /*@type=int*/ x = c?. /*@target=C::f*/ f();
+  c?. /*@target=C::f*/ f();
 }
 
 main() {}

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

@@ -11,5 +11,6 @@ class C extends core::Object {
 }
 static method g(self::C c) → dynamic {
   dynamic x = let final dynamic #t1 = c in #t1.==(null) ? null : #t1.f();
+  let final dynamic #t2 = c in #t2.==(null) ? null : #t2.f();
 }
 static method main() → dynamic {}

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

@@ -11,5 +11,6 @@ class C extends core::Object {
 }
 static method g(self::C c) → dynamic {
   core::int x = let final self::C #t1 = c in #t1.==(null) ?{core::int} null : #t1.{self::C::f}();
+  let final self::C #t2 = c in #t2.==(null) ?{core::int} null : #t2.{self::C::f}();
 }
 static method main() → dynamic {}

pkg/front_end/testcases/inference/null_aware_property_get.dart

@@ -11,6 +11,7 @@ class C {
 
 void f(C c) {
   var /*@type=int*/ x = c?. /*@target=C::x*/ x;
+  c?. /*@target=C::x*/ x;
 }
 
 main() {}

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

@@ -10,5 +10,6 @@ class C extends core::Object {
 }
 static method f(self::C c) → void {
   dynamic x = let final dynamic #t1 = c in #t1.==(null) ? null : #t1.x;
+  let final dynamic #t2 = c in #t2.==(null) ? null : #t2.x;
 }
 static method main() → dynamic {}

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 {}