Make ?? a compile-time constant operator.

pkg/compiler/lib/src/constant_system_dart.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.dart;
 
 import 'compiler.dart' show
     Compiler;
 import 'constants/constant_system.dart';
 import 'constants/values.dart';
@@ skipping 314 matching lines @@
   BoolConstantValue fold(ConstantValue left, ConstantValue right) {
     // In order to preserve runtime semantics which says that NaN !== NaN don't
     // constant fold NaN === NaN. Otherwise the output depends on inlined
     // variables and other optimizations.
     if (left.isNaN && right.isNaN) return null;
     return DART_CONSTANT_SYSTEM.createBool(left == right);
   }
   apply(left, right) => identical(left, right);
 }
 
+class IfNullOperation implements BinaryOperation {
+  final String name = '??';
+  const IfNullOperation();
+  ConstantValue fold(ConstantValue left, ConstantValue right) {
+    if (left.isNull) return right;
+    return left;
+  }
+  apply(left, right) => left ?? right;
+}
+
 abstract class CodeUnitAtOperation implements BinaryOperation {
   final String name = 'charCodeAt';
   const CodeUnitAtOperation();
   apply(left, right) => left.codeUnitAt(right);
 }
 
 class CodeUnitAtConstantOperation extends CodeUnitAtOperation {
   const CodeUnitAtConstantOperation();
   ConstantValue fold(ConstantValue left, ConstantValue right) {
     // 'a'.codeUnitAt(0) is not a constant expression.
@@ skipping 30 matching lines @@
   final bitNot = const BitNotOperation();
   final bitOr = const BitOrOperation();
   final bitXor = const BitXorOperation();
   final booleanAnd = const BooleanAndOperation();
   final booleanOr = const BooleanOrOperation();
   final divide = const DivideOperation();
   final equal = const EqualsOperation();
   final greaterEqual = const GreaterEqualOperation();
   final greater = const GreaterOperation();
   final identity = const IdentityOperation();
+  final ifNull = const IfNullOperation();
   final lessEqual = const LessEqualOperation();
   final less = const LessOperation();
   final modulo = const ModuloOperation();
   final multiply = const MultiplyOperation();
   final negate = const NegateOperation();
   final not = const NotOperation();
   final shiftLeft = const ShiftLeftOperation();
   final shiftRight = const ShiftRightOperation();
   final subtract = const SubtractOperation();
   final truncatingDivide = const TruncatingDivideOperation();
@@ skipping 45 matching lines @@
   bool isInt(ConstantValue constant) => constant.isInt;
   bool isDouble(ConstantValue constant) => constant.isDouble;
   bool isString(ConstantValue constant) => constant.isString;
   bool isBool(ConstantValue constant) => constant.isBool;
   bool isNull(ConstantValue constant) => constant.isNull;
 
   bool isSubtype(DartTypes types, DartType s, DartType t) {
     return types.isSubtype(s, t);
   }
 }