Handle constant ?? expressions in constructors.

pkg/compiler/lib/src/constants/constant_constructors.dart

Index: pkg/compiler/lib/src/constants/constant_constructors.dart
diff --git a/pkg/compiler/lib/src/constants/constant_constructors.dart b/pkg/compiler/lib/src/constants/constant_constructors.dart
index 3dceb5981154c919a4954fed7e238248ad4a3595..4628f28629ef34f1e98467950eef14de9372289c 100644
--- a/pkg/compiler/lib/src/constants/constant_constructors.dart
+++ b/pkg/compiler/lib/src/constants/constant_constructors.dart
@@ -339,4 +339,10 @@ class ConstantConstructorComputer extends SemanticVisitor
   ConstantExpression visitNamedArgument(NamedArgument node) {
     return apply(node.expression);
   }
+
+  @override
+  ConstantExpression visitIfNull(Send node, Node left, Node right, _) {
+    return new BinaryConstantExpression(
+        apply(left), BinaryOperator.IF_NULL, apply(right));
+  }
 }