Add visitor methods specific to ??= to SemanticSendVisitor.

pkg/compiler/lib/src/resolution/members.dart

Index: pkg/compiler/lib/src/resolution/members.dart
diff --git a/pkg/compiler/lib/src/resolution/members.dart b/pkg/compiler/lib/src/resolution/members.dart
index 57081995b7fae47de9d6aecd0d4002a40533a5a2..5a80a1089dbce729924aae5d33567fac29808a3a 100644
--- a/pkg/compiler/lib/src/resolution/members.dart
+++ b/pkg/compiler/lib/src/resolution/members.dart
@@ -3459,8 +3459,12 @@ class ResolverVisitor extends MappingVisitor<ResolutionResult> {
         registry.registerDynamicInvocation(
             new UniverseSelector(operatorSelector, null));
 
-        SendStructure sendStructure =
-            new CompoundStructure(semantics, operator);
+        SendStructure sendStructure;
+        if (operator.kind == AssignmentOperatorKind.IF_NULL) {
+          sendStructure = new SetIfNullStructure(semantics);
+        } else {
+          sendStructure = new CompoundStructure(semantics, operator);
+        }
         registry.registerSendStructure(node, sendStructure);
       }
     }