Handle constant ?? expressions in constructors.

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

 // Copyright (c) 2015, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 // TODO(johnniwinther): Remove this library when all constant constructors are
 // computed during resolution.
 library dart2js.constants.constant_constructors;
 
 import '../common.dart';
 import '../dart_types.dart';
@@ skipping 321 matching lines @@
       throw new UnsupportedError("Unexpected function call: $function");
     }
     return new IdenticalConstantExpression(
         apply(arguments.nodes.head), apply(arguments.nodes.tail.head));
   }
 
   @override
   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));
+  }
 }