Add visitor methods specific to ??= to SemanticSendVisitor.

tests/compiler/dart2js/semantic_visitor_test.dart

Index: tests/compiler/dart2js/semantic_visitor_test.dart
diff --git a/tests/compiler/dart2js/semantic_visitor_test.dart b/tests/compiler/dart2js/semantic_visitor_test.dart
index 969b95d2849947f0b1c6d7076e2fcb5307f785f1..37d83bb24b15753b6846fbdcd5787f88e168b469 100644
--- a/tests/compiler/dart2js/semantic_visitor_test.dart
+++ b/tests/compiler/dart2js/semantic_visitor_test.dart
@@ -182,17 +182,21 @@ const List<VisitKind> UNTESTABLE_KINDS = const <VisitKind>[
   // A final field shadowing a non-final field is currently not supported in
   // resolution.
   VisitKind.VISIT_SUPER_FIELD_FIELD_COMPOUND,
+  VisitKind.VISIT_SUPER_FIELD_FIELD_SET_IF_NULL,
   VisitKind.VISIT_SUPER_FIELD_FIELD_PREFIX,
   VisitKind.VISIT_SUPER_FIELD_FIELD_POSTFIX,
   // Combination of method and setter with the same name is currently not
   // supported by the element model.
   VisitKind.VISIT_STATIC_METHOD_SETTER_COMPOUND,
+  VisitKind.VISIT_STATIC_METHOD_SETTER_SET_IF_NULL,
   VisitKind.VISIT_STATIC_METHOD_SETTER_PREFIX,
   VisitKind.VISIT_STATIC_METHOD_SETTER_POSTFIX,
   VisitKind.VISIT_TOP_LEVEL_METHOD_SETTER_COMPOUND,
+  VisitKind.VISIT_TOP_LEVEL_METHOD_SETTER_SET_IF_NULL,
   VisitKind.VISIT_TOP_LEVEL_METHOD_SETTER_PREFIX,
   VisitKind.VISIT_TOP_LEVEL_METHOD_SETTER_POSTFIX,
   VisitKind.VISIT_SUPER_METHOD_SETTER_COMPOUND,
+  VisitKind.VISIT_SUPER_METHOD_SETTER_SET_IF_NULL,
   VisitKind.VISIT_SUPER_METHOD_SETTER_PREFIX,
   VisitKind.VISIT_SUPER_METHOD_SETTER_POSTFIX,
   // The only undefined unary, `+`, is currently handled and skipped in the
@@ -371,10 +375,12 @@ enum VisitKind {
   VISIT_PARAMETER_SET,
   VISIT_PARAMETER_INVOKE,
   VISIT_PARAMETER_COMPOUND,
+  VISIT_PARAMETER_SET_IF_NULL,
   VISIT_PARAMETER_PREFIX,
   VISIT_PARAMETER_POSTFIX,
   VISIT_FINAL_PARAMETER_SET,
   VISIT_FINAL_PARAMETER_COMPOUND,
+  VISIT_FINAL_PARAMETER_SET_IF_NULL,
   VISIT_FINAL_PARAMETER_PREFIX,
   VISIT_FINAL_PARAMETER_POSTFIX,
 
@@ -382,12 +388,14 @@ enum VisitKind {
   VISIT_LOCAL_VARIABLE_SET,
   VISIT_LOCAL_VARIABLE_INVOKE,
   VISIT_LOCAL_VARIABLE_COMPOUND,
+  VISIT_LOCAL_VARIABLE_SET_IF_NULL,
   VISIT_LOCAL_VARIABLE_PREFIX,
   VISIT_LOCAL_VARIABLE_POSTFIX,
   VISIT_LOCAL_VARIABLE_DECL,
   VISIT_LOCAL_CONSTANT_DECL,
   VISIT_FINAL_LOCAL_VARIABLE_SET,
   VISIT_FINAL_LOCAL_VARIABLE_COMPOUND,
+  VISIT_FINAL_LOCAL_VARIABLE_SET_IF_NULL,
   VISIT_FINAL_LOCAL_VARIABLE_PREFIX,
   VISIT_FINAL_LOCAL_VARIABLE_POSTFIX,
 
@@ -398,6 +406,7 @@ enum VisitKind {
   VISIT_CLOSURE_DECL,
   VISIT_LOCAL_FUNCTION_SET,
   VISIT_LOCAL_FUNCTION_COMPOUND,
+  VISIT_LOCAL_FUNCTION_SET_IF_NULL,
   VISIT_LOCAL_FUNCTION_PREFIX,
   VISIT_LOCAL_FUNCTION_POSTFIX,
 
@@ -405,6 +414,7 @@ enum VisitKind {
   VISIT_STATIC_FIELD_SET,
   VISIT_STATIC_FIELD_INVOKE,
   VISIT_STATIC_FIELD_COMPOUND,
+  VISIT_STATIC_FIELD_SET_IF_NULL,
   VISIT_STATIC_FIELD_PREFIX,
   VISIT_STATIC_FIELD_POSTFIX,
   VISIT_STATIC_FIELD_DECL,
@@ -419,7 +429,9 @@ enum VisitKind {
   VISIT_STATIC_SETTER_INVOKE,
 
   VISIT_STATIC_GETTER_SETTER_COMPOUND,
+  VISIT_STATIC_GETTER_SETTER_SET_IF_NULL,
   VISIT_STATIC_METHOD_SETTER_COMPOUND,
+  VISIT_STATIC_METHOD_SETTER_SET_IF_NULL,
   VISIT_STATIC_GETTER_SETTER_PREFIX,
   VISIT_STATIC_GETTER_SETTER_POSTFIX,
 
@@ -428,6 +440,7 @@ enum VisitKind {
 
   VISIT_FINAL_STATIC_FIELD_SET,
   VISIT_STATIC_FINAL_FIELD_COMPOUND,
+  VISIT_STATIC_FINAL_FIELD_SET_IF_NULL,
   VISIT_STATIC_FINAL_FIELD_POSTFIX,
   VISIT_STATIC_FINAL_FIELD_PREFIX,
 
@@ -440,8 +453,11 @@ enum VisitKind {
   VISIT_STATIC_METHOD_SETTER_POSTFIX,
 
   VISIT_UNRESOLVED_STATIC_GETTER_COMPOUND,
+  VISIT_UNRESOLVED_STATIC_GETTER_SET_IF_NULL,
   VISIT_UNRESOLVED_STATIC_SETTER_COMPOUND,
+  VISIT_UNRESOLVED_STATIC_SETTER_SET_IF_NULL,
   VISIT_STATIC_METHOD_COMPOUND,
+  VISIT_STATIC_METHOD_SET_IF_NULL,
   VISIT_UNRESOLVED_STATIC_GETTER_PREFIX,
   VISIT_UNRESOLVED_STATIC_SETTER_PREFIX,
   VISIT_STATIC_METHOD_PREFIX,
@@ -454,11 +470,13 @@ enum VisitKind {
   VISIT_TOP_LEVEL_FIELD_INVOKE,
   VISIT_FINAL_TOP_LEVEL_FIELD_SET,
   VISIT_TOP_LEVEL_FIELD_COMPOUND,
+  VISIT_TOP_LEVEL_FIELD_SET_IF_NULL,
   VISIT_TOP_LEVEL_FIELD_PREFIX,
   VISIT_TOP_LEVEL_FIELD_POSTFIX,
   VISIT_TOP_LEVEL_FIELD_DECL,
   VISIT_TOP_LEVEL_CONSTANT_DECL,
   VISIT_TOP_LEVEL_FINAL_FIELD_COMPOUND,
+  VISIT_TOP_LEVEL_FINAL_FIELD_SET_IF_NULL,
   VISIT_TOP_LEVEL_FINAL_FIELD_POSTFIX,
   VISIT_TOP_LEVEL_FINAL_FIELD_PREFIX,
 
@@ -469,6 +487,7 @@ enum VisitKind {
   VISIT_TOP_LEVEL_SETTER_SET,
   VISIT_TOP_LEVEL_SETTER_INVOKE,
   VISIT_TOP_LEVEL_GETTER_SETTER_COMPOUND,
+  VISIT_TOP_LEVEL_GETTER_SETTER_SET_IF_NULL,
   VISIT_TOP_LEVEL_GETTER_SETTER_PREFIX,
   VISIT_TOP_LEVEL_GETTER_SETTER_POSTFIX,
   VISIT_TOP_LEVEL_GETTER_DECL,
@@ -480,12 +499,16 @@ enum VisitKind {
   VISIT_TOP_LEVEL_FUNCTION_INCOMPATIBLE_INVOKE,
   VISIT_TOP_LEVEL_FUNCTION_DECL,
   VISIT_TOP_LEVEL_METHOD_SETTER_COMPOUND,
+  VISIT_TOP_LEVEL_METHOD_SETTER_SET_IF_NULL,
   VISIT_TOP_LEVEL_METHOD_SETTER_PREFIX,
   VISIT_TOP_LEVEL_METHOD_SETTER_POSTFIX,
 
   VISIT_UNRESOLVED_TOP_LEVEL_GETTER_COMPOUND,
+  VISIT_UNRESOLVED_TOP_LEVEL_GETTER_SET_IF_NULL,
   VISIT_UNRESOLVED_TOP_LEVEL_SETTER_COMPOUND,
+  VISIT_UNRESOLVED_TOP_LEVEL_SETTER_SET_IF_NULL,
   VISIT_TOP_LEVEL_METHOD_COMPOUND,
+  VISIT_TOP_LEVEL_METHOD_SET_IF_NULL,
   VISIT_UNRESOLVED_TOP_LEVEL_GETTER_PREFIX,
   VISIT_UNRESOLVED_TOP_LEVEL_SETTER_PREFIX,
   VISIT_TOP_LEVEL_METHOD_PREFIX,
@@ -497,6 +520,7 @@ enum VisitKind {
   VISIT_DYNAMIC_PROPERTY_SET,
   VISIT_DYNAMIC_PROPERTY_INVOKE,
   VISIT_DYNAMIC_PROPERTY_COMPOUND,
+  VISIT_DYNAMIC_PROPERTY_SET_IF_NULL,
   VISIT_DYNAMIC_PROPERTY_PREFIX,
   VISIT_DYNAMIC_PROPERTY_POSTFIX,
 
@@ -507,6 +531,7 @@ enum VisitKind {
   VISIT_THIS_PROPERTY_SET,
   VISIT_THIS_PROPERTY_INVOKE,
   VISIT_THIS_PROPERTY_COMPOUND,
+  VISIT_THIS_PROPERTY_SET_IF_NULL,
   VISIT_THIS_PROPERTY_PREFIX,
   VISIT_THIS_PROPERTY_POSTFIX,
 
@@ -515,12 +540,15 @@ enum VisitKind {
   VISIT_FINAL_SUPER_FIELD_SET,
   VISIT_SUPER_FIELD_INVOKE,
   VISIT_SUPER_FIELD_COMPOUND,
+  VISIT_SUPER_FIELD_SET_IF_NULL,
   VISIT_SUPER_FIELD_PREFIX,
   VISIT_SUPER_FIELD_POSTFIX,
   VISIT_SUPER_FINAL_FIELD_COMPOUND,
+  VISIT_SUPER_FINAL_FIELD_SET_IF_NULL,
   VISIT_SUPER_FINAL_FIELD_PREFIX,
   VISIT_SUPER_FINAL_FIELD_POSTFIX,
   VISIT_SUPER_FIELD_FIELD_COMPOUND,
+  VISIT_SUPER_FIELD_FIELD_SET_IF_NULL,
   VISIT_SUPER_FIELD_FIELD_PREFIX,
   VISIT_SUPER_FIELD_FIELD_POSTFIX,
 
@@ -531,8 +559,11 @@ enum VisitKind {
   VISIT_SUPER_SETTER_SET,
   VISIT_SUPER_SETTER_INVOKE,
   VISIT_SUPER_GETTER_SETTER_COMPOUND,
+  VISIT_SUPER_GETTER_SETTER_SET_IF_NULL,
   VISIT_SUPER_GETTER_FIELD_COMPOUND,
+  VISIT_SUPER_GETTER_FIELD_SET_IF_NULL,
   VISIT_SUPER_FIELD_SETTER_COMPOUND,
+  VISIT_SUPER_FIELD_SETTER_SET_IF_NULL,
   VISIT_SUPER_GETTER_SETTER_PREFIX,
   VISIT_SUPER_GETTER_FIELD_PREFIX,
   VISIT_SUPER_FIELD_SETTER_PREFIX,
@@ -545,9 +576,11 @@ enum VisitKind {
   VISIT_SUPER_METHOD_INVOKE,
   VISIT_SUPER_METHOD_INCOMPATIBLE_INVOKE,
   VISIT_SUPER_METHOD_SETTER_COMPOUND,
+  VISIT_SUPER_METHOD_SETTER_SET_IF_NULL,
   VISIT_SUPER_METHOD_SETTER_PREFIX,
   VISIT_SUPER_METHOD_SETTER_POSTFIX,
   VISIT_SUPER_METHOD_COMPOUND,
+  VISIT_SUPER_METHOD_SET_IF_NULL,
   VISIT_SUPER_METHOD_PREFIX,
   VISIT_SUPER_METHOD_POSTFIX,
 
@@ -572,7 +605,9 @@ enum VisitKind {
   VISIT_SUPER_NOT_EQUALS,
   VISIT_SUPER_INDEX_PREFIX,
   VISIT_UNRESOLVED_SUPER_GETTER_COMPOUND,
+  VISIT_UNRESOLVED_SUPER_GETTER_SET_IF_NULL,
   VISIT_UNRESOLVED_SUPER_SETTER_COMPOUND,
+  VISIT_UNRESOLVED_SUPER_SETTER_SET_IF_NULL,
   VISIT_UNRESOLVED_SUPER_GETTER_PREFIX,
   VISIT_UNRESOLVED_SUPER_SETTER_PREFIX,
   VISIT_UNRESOLVED_SUPER_INDEX_PREFIX,
@@ -586,6 +621,7 @@ enum VisitKind {
   VISIT_UNRESOLVED_SUPER_SETTER_INDEX_POSTFIX,
 
   VISIT_UNRESOLVED_SUPER_COMPOUND,
+  VISIT_UNRESOLVED_SUPER_SET_IF_NULL,
   VISIT_UNRESOLVED_SUPER_PREFIX,
   VISIT_UNRESOLVED_SUPER_POSTFIX,
 
@@ -600,6 +636,7 @@ enum VisitKind {
   VISIT_CLASS_TYPE_LITERAL_SET,
   VISIT_CLASS_TYPE_LITERAL_INVOKE,
   VISIT_CLASS_TYPE_LITERAL_COMPOUND,
+  VISIT_CLASS_TYPE_LITERAL_SET_IF_NULL,
   VISIT_CLASS_TYPE_LITERAL_PREFIX,
   VISIT_CLASS_TYPE_LITERAL_POSTFIX,
 
@@ -607,6 +644,7 @@ enum VisitKind {
   VISIT_TYPEDEF_TYPE_LITERAL_SET,
   VISIT_TYPEDEF_TYPE_LITERAL_INVOKE,
   VISIT_TYPEDEF_TYPE_LITERAL_COMPOUND,
+  VISIT_TYPEDEF_TYPE_LITERAL_SET_IF_NULL,
   VISIT_TYPEDEF_TYPE_LITERAL_PREFIX,
   VISIT_TYPEDEF_TYPE_LITERAL_POSTFIX,
 
@@ -614,6 +652,7 @@ enum VisitKind {
   VISIT_TYPE_VARIABLE_TYPE_LITERAL_SET,
   VISIT_TYPE_VARIABLE_TYPE_LITERAL_INVOKE,
   VISIT_TYPE_VARIABLE_TYPE_LITERAL_COMPOUND,
+  VISIT_TYPE_VARIABLE_TYPE_LITERAL_SET_IF_NULL,
   VISIT_TYPE_VARIABLE_TYPE_LITERAL_PREFIX,
   VISIT_TYPE_VARIABLE_TYPE_LITERAL_POSTFIX,
 
@@ -621,6 +660,7 @@ enum VisitKind {
   VISIT_DYNAMIC_TYPE_LITERAL_SET,
   VISIT_DYNAMIC_TYPE_LITERAL_INVOKE,
   VISIT_DYNAMIC_TYPE_LITERAL_COMPOUND,
+  VISIT_DYNAMIC_TYPE_LITERAL_SET_IF_NULL,
   VISIT_DYNAMIC_TYPE_LITERAL_PREFIX,
   VISIT_DYNAMIC_TYPE_LITERAL_POSTFIX,
 
@@ -682,6 +722,7 @@ enum VisitKind {
   VISIT_NAMED_INITIALIZING_FORMAL_DECL,
 
   VISIT_UNRESOLVED_COMPOUND,
+  VISIT_UNRESOLVED_SET_IF_NULL,
   VISIT_UNRESOLVED_PREFIX,
   VISIT_UNRESOLVED_POSTFIX,
 
@@ -690,6 +731,7 @@ enum VisitKind {
   VISIT_IF_NOT_NULL_DYNAMIC_PROPERTY_SET,
   VISIT_IF_NOT_NULL_DYNAMIC_PROPERTY_INVOKE,
   VISIT_IF_NOT_NULL_DYNAMIC_PROPERTY_COMPOUND,
+  VISIT_IF_NOT_NULL_DYNAMIC_PROPERTY_SET_IF_NULL,
   VISIT_IF_NOT_NULL_DYNAMIC_PROPERTY_PREFIX,
   VISIT_IF_NOT_NULL_DYNAMIC_PROPERTY_POSTFIX,
 
@@ -702,6 +744,7 @@ enum VisitKind {
   ERROR_INVALID_PREFIX,
   ERROR_INVALID_POSTFIX,
   ERROR_INVALID_COMPOUND,
+  ERROR_INVALID_SET_IF_NULL,
   ERROR_INVALID_UNARY,
   ERROR_INVALID_EQUALS,
   ERROR_INVALID_NOT_EQUALS,