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

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

 // Copyright (c) 2017, 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.md file.
 
 import 'package:front_end/src/fasta/type_inference/type_inference_engine.dart';
 import 'package:kernel/ast.dart';
 
 /// Base class for [TypeInferenceListener] that defines the API for debugging.
 ///
 /// By default no debug info is printed.  To enable debug printing, mix in
@@ skipping 145 matching lines @@
       debugStatementExit('functionDeclaration', statement);
 
   bool functionExpressionEnter(
           FunctionExpression expression, DartType typeContext) =>
       debugExpressionEnter("functionExpression", expression, typeContext);
 
   void functionExpressionExit(
           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);
 
   void ifStatementExit(IfStatement statement) =>
       debugStatementExit('ifStatement', statement);
 
   bool indexAssignEnter(Expression expression, DartType typeContext) =>
       debugExpressionEnter("indexAssign", expression, typeContext);
 
   void indexAssignExit(Expression expression, DartType inferredType) =>
@@ skipping 138 matching lines @@
 
   void variableSetExit(VariableSet expression, DartType inferredType) =>
       debugExpressionExit("variableSet", expression, inferredType);
 
   void yieldStatementEnter(YieldStatement statement) =>
       debugStatementEnter('yieldStatement', statement);
 
   void yieldStatementExit(YieldStatement statement) =>
       debugStatementExit('yieldStatement', statement);
 }