Make ?? a compile-time constant operator.

pkg/compiler/lib/src/js_backend/constant_system_javascript.dart

 // Copyright (c) 2012, 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.
 
 library dart2js.constant_system.js;
 
 import '../compiler.dart' show
     Compiler;
 import '../constants/constant_system.dart';
 import '../constants/values.dart';
@@ skipping 173 matching lines @@
   final bitOr = const JavaScriptBinaryBitOperation(const BitOrOperation());
   final bitXor = const JavaScriptBinaryBitOperation(const BitXorOperation());
   final booleanAnd = const BooleanAndOperation();
   final booleanOr = const BooleanOrOperation();
   final divide =
       const JavaScriptBinaryArithmeticOperation(const DivideOperation());
   final equal = const EqualsOperation();
   final greaterEqual = const GreaterEqualOperation();
   final greater = const GreaterOperation();
   final identity = const JavaScriptIdentityOperation();
+  final ifNull = const IfNullOperation();
   final lessEqual = const LessEqualOperation();
   final less = const LessOperation();
   final modulo =
       const JavaScriptBinaryArithmeticOperation(const ModuloOperation());
   final multiply =
       const JavaScriptBinaryArithmeticOperation(const MultiplyOperation());
   final negate = const JavaScriptNegateOperation();
   final not = const NotOperation();
   final shiftLeft =
       const JavaScriptBinaryBitOperation(const ShiftLeftOperation());
@@ skipping 176 matching lines @@
       result.add(keyList);
     } else {
       // Add the keys individually to avoid generating an unused list constant
       // for the keys.
       result.addAll(keys);
     }
     result.addAll(values);
     return result;
   }
 }