Make ?? a compile-time constant operator.

pkg/compiler/lib/src/constant_system_dart.dart

Index: pkg/compiler/lib/src/constant_system_dart.dart
diff --git a/pkg/compiler/lib/src/constant_system_dart.dart b/pkg/compiler/lib/src/constant_system_dart.dart
index 4a15ac9aff7cba10487fe95cb9a8086619ee39ea..c2c819ed7c53988d3abc9be1107861a5e94b6eea 100644
--- a/pkg/compiler/lib/src/constant_system_dart.dart
+++ b/pkg/compiler/lib/src/constant_system_dart.dart
@@ -332,6 +332,16 @@ class IdentityOperation implements BinaryOperation {
   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();
@@ -382,6 +392,7 @@ class DartConstantSystem extends ConstantSystem {
   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();