Fix incorrect nullability inference for ??

pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart

Index: pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
diff --git a/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart b/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
index 45330349f4329dbccb85904126868b4a34b9a5fd..d6a7f07842a086387805124d90c5937a6f3e622d 100644
--- a/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
+++ b/pkg/dev_compiler/lib/src/compiler/nullable_type_inference.dart
@@ -59,7 +59,8 @@ abstract class NullableTypeInference {
     // resulting value if that becomes an issue, so we maintain the invariant
     // that each node is visited once.
     Element element = null;
-    if (expr is PropertyAccess) {
+    if (expr is PropertyAccess &&
+        expr.operator?.type != TokenType.QUESTION_PERIOD) {
       element = expr.propertyName.staticElement;
     } else if (expr is Identifier) {
       element = expr.staticElement;