Implement type inference for null-aware property gets.

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 219 matching lines @@
 
   void nullLiteralExit(NullLiteral expression, DartType inferredType) =>
       debugExpressionExit("nullLiteral", expression, inferredType);
 
   bool propertyAssignEnter(Expression expression, DartType typeContext) =>
       debugExpressionEnter("propertyAssign", expression, typeContext);
 
   void propertyAssignExit(Expression expression, DartType inferredType) =>
       debugExpressionExit("propertyAssign", expression, inferredType);
 
-  bool propertyGetEnter(PropertyGet expression, DartType typeContext) =>
+  bool propertyGetEnter(Expression expression, DartType typeContext) =>
       debugExpressionEnter("propertyGet", expression, typeContext);
 
-  void propertyGetExit(PropertyGet expression, DartType inferredType) =>
+  void propertyGetExit(Expression expression, DartType inferredType) =>
       debugExpressionExit("propertyGet", expression, inferredType);
 
   bool propertySetEnter(PropertySet expression, DartType typeContext) =>
       debugExpressionEnter("propertySet", expression, typeContext);
 
   void propertySetExit(PropertySet expression, DartType inferredType) =>
       debugExpressionExit("propertySet", expression, inferredType);
 
   void recordDependency(FieldNode fieldNode) => debugDependency(fieldNode);
 
@@ skipping 66 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);
 }