Add visitor methods specific to ??= to SemanticSendVisitor.

pkg/compiler/lib/src/inferrer/inferrer_visitor.dart

Index: pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
diff --git a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
index 1f35e802e12a06779ffd7b80e308792077ae4db1..e542d49d2945624e2b259de679d8abb5409b6096 100644
--- a/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
+++ b/pkg/compiler/lib/src/inferrer/inferrer_visitor.dart
@@ -692,6 +692,7 @@ abstract class InferrerVisitor<T, E extends MinimalInferrerEngine<T>>
     with SendResolverMixin,
          SemanticSendResolvedMixin<T, dynamic>,
          CompoundBulkMixin<T, dynamic>,
+         SetIfNullBulkMixin<T, dynamic>,
          PrefixBulkMixin<T, dynamic>,
          PostfixBulkMixin<T, dynamic>,
          ErrorBulkMixin<T, dynamic>,
@@ -773,6 +774,11 @@ abstract class InferrerVisitor<T, E extends MinimalInferrerEngine<T>>
   }
 
   @override
+  T bulkHandleSetIfNull(SendSet node, _) {
+    return handleSendSet(node);
+  }
+
+  @override
   T bulkHandlePrefix(SendSet node, _) {
     return handleSendSet(node);
   }