Make ?? a compile-time constant operator.

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

 // Copyright (c) 2014, 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.constants.expressions;
 
 import '../common.dart';
 import '../constants/constant_system.dart';
 import '../core_types.dart';
 import '../dart_types.dart';
@@ skipping 786 matching lines @@
     BinaryOperatorKind.ADD: 12,
     BinaryOperatorKind.SUB: 12,
     BinaryOperatorKind.MUL: 13,
     BinaryOperatorKind.DIV: 13,
     BinaryOperatorKind.IDIV: 13,
     BinaryOperatorKind.GT: 7,
     BinaryOperatorKind.LT: 7,
     BinaryOperatorKind.GTEQ: 7,
     BinaryOperatorKind.LTEQ: 7,
     BinaryOperatorKind.MOD: 13,
+    BinaryOperatorKind.IF_NULL: 3,
   };
 }
 
 /// A constant identical invocation like `identical(a, b)`.
 class IdenticalConstantExpression extends ConstantExpression {
   final ConstantExpression left;
   final ConstantExpression right;
 
   IdenticalConstantExpression(this.left, this.right);
 
@@ skipping 781 matching lines @@
     sb.write('const String.fromEnvironment(');
     visit(exp.name);
     if (exp.defaultValue != null) {
       sb.write(', defaultValue: ');
       visit(exp.defaultValue);
     }
     sb.write(')');
   }
 
   String toString() => sb.toString();
-}
+}