Make ?? a compile-time constant operator.

pkg/compiler/lib/src/constants/constant_system.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;
 
 import '../dart_types.dart';
 import '../compiler.dart' show
     Compiler;
 import '../resolution/operators.dart';
@@ skipping 25 matching lines @@
   UnaryOperation get bitNot;
   BinaryOperation get bitOr;
   BinaryOperation get bitXor;
   BinaryOperation get booleanAnd;
   BinaryOperation get booleanOr;
   BinaryOperation get divide;
   BinaryOperation get equal;
   BinaryOperation get greaterEqual;
   BinaryOperation get greater;
   BinaryOperation get identity;
+  BinaryOperation get ifNull;
   BinaryOperation get lessEqual;
   BinaryOperation get less;
   BinaryOperation get modulo;
   BinaryOperation get multiply;
   UnaryOperation get negate;
   UnaryOperation get not;
   BinaryOperation get shiftLeft;
   BinaryOperation get shiftRight;
   BinaryOperation get subtract;
   BinaryOperation get truncatingDivide;
@@ skipping 55 matching lines @@
       case BinaryOperatorKind.XOR:   return bitXor;
       case BinaryOperatorKind.LOGICAL_OR:  return booleanOr;
       case BinaryOperatorKind.LOGICAL_AND:  return booleanAnd;
       case BinaryOperatorKind.SHL:  return shiftLeft;
       case BinaryOperatorKind.SHR:  return shiftRight;
       case BinaryOperatorKind.LT:   return less;
       case BinaryOperatorKind.LTEQ:  return lessEqual;
       case BinaryOperatorKind.GT:   return greater;
       case BinaryOperatorKind.GTEQ:  return greaterEqual;
       case BinaryOperatorKind.EQ:  return equal;
+      case BinaryOperatorKind.IF_NULL: return ifNull;
       default:    return null;
     }
   }
 }