Handle ?? in Constantifier

tests/compiler/dart2js/kernel/impact_test.dart

Index: tests/compiler/dart2js/kernel/impact_test.dart
diff --git a/tests/compiler/dart2js/kernel/impact_test.dart b/tests/compiler/dart2js/kernel/impact_test.dart
index 482a000995f968f46b9aa30ae4afb6a793251b8f..ef04e1eb8898a6ec242d5a13589490a480aa825b 100644
--- a/tests/compiler/dart2js/kernel/impact_test.dart
+++ b/tests/compiler/dart2js/kernel/impact_test.dart
@@ -57,6 +57,7 @@ main() {
   testSymbol();
   testConstSymbol();
   testComplexConstSymbol();
+  testIfNullConstSymbol();
   testTypeLiteral();
   testBoolFromEnvironment();
   testEmptyListLiteral();
@@ -234,6 +235,8 @@ const complexSymbolField =
     complexSymbolField1 ? complexSymbolField2 : complexSymbolField3;
 testComplexConstSymbol() => const Symbol(complexSymbolField);
 
+testIfNullConstSymbol() => const Symbol(null ?? 'foo');
+
 testTypeLiteral() => Object;
 testBoolFromEnvironment() => const bool.fromEnvironment('FOO');
 testEmptyListLiteral() => [];