Add support for null-aware operators to the CPS-IR.

pkg/compiler/lib/src/resolution/operators.dart

Index: pkg/compiler/lib/src/resolution/operators.dart
diff --git a/pkg/compiler/lib/src/resolution/operators.dart b/pkg/compiler/lib/src/resolution/operators.dart
index 594e0a0c3077747c7d7d4e74b1ceb01ec6425058..4b9ec51968192cb284629641b327e0e52b5e27b7 100644
--- a/pkg/compiler/lib/src/resolution/operators.dart
+++ b/pkg/compiler/lib/src/resolution/operators.dart
@@ -170,7 +170,7 @@ class BinaryOperator {
 
   /// The if-null ?? operator.
   static const BinaryOperator IF_NULL =
-      const _LogicalOperator(BinaryOperatorKind.IF_NULL, '??');
+      const _IfNullOperator(BinaryOperatorKind.IF_NULL, '??');
 
   static BinaryOperator parse(String value) {
     switch (value) {
@@ -221,6 +221,16 @@ class _LogicalOperator extends BinaryOperator {
   String get selectorName => null;
 }
 
+/// The operators ?? is not user definable.
+class _IfNullOperator extends BinaryOperator {
+  const _IfNullOperator(BinaryOperatorKind kind, String name)
+      : super._(kind, name);
+
+  bool get isUserDefinable => false;
+
+  String get selectorName => '??';
+}
+
 enum AssignmentOperatorKind {
   ASSIGN,
   IF_NULL,