Implement type inference for if-null expressions (a ?? b).

pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart

Index: pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
diff --git a/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart b/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
index 4c4ee5e8d7038e20101929f107d9de245f5bab09..ff3e755383adb4edb0158cbdeead38c0c7146e73 100644
--- a/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
+++ b/pkg/front_end/lib/src/fasta/type_inference/type_inference_listener.dart
@@ -163,6 +163,12 @@ class TypeInferenceListener
           FunctionExpression expression, DartType inferredType) =>
       debugExpressionExit("functionExpression", expression, inferredType);
 
+  bool ifNullEnter(Expression expression, DartType typeContext) =>
+      debugExpressionEnter('ifNull', expression, typeContext);
+
+  void ifNullExit(Expression expression, DartType inferredType) =>
+      debugExpressionExit('ifNull', expression, inferredType);
+
   void ifStatementEnter(IfStatement statement) =>
       debugStatementEnter('ifStatement', statement);