Infer AsExpression types.

pkg/front_end/lib/src/fasta/kernel/kernel_shadow_ast.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 file.
 
 /// This file declares a "shadow hierarchy" of concrete classes which extend
 /// the kernel class hierarchy, adding methods and fields needed by the
 /// BodyBuilder.
 ///
 /// Instances of these classes may be created using the factory methods in
 /// `ast_factory.dart`.
@@ skipping 12 matching lines @@
 import 'package:front_end/src/fasta/type_inference/type_promotion.dart';
 import 'package:kernel/ast.dart';
 
 /// Shadow object for [AsExpression].
 class KernelAsExpression extends AsExpression implements KernelExpression {
   KernelAsExpression(Expression operand, DartType type) : super(operand, type);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
-    // TODO(scheglov): implement.
-    return typeNeeded ? const DynamicType() : null;
+    return inferrer.inferAsExpression(typeContext, typeNeeded, operand, type);
   }
 }
 
 /// Shadow object for [AwaitExpression].
 class KernelAwaitExpression extends AwaitExpression
     implements KernelExpression {
   KernelAwaitExpression(Expression operand) : super(operand);
 
   @override
   DartType _inferExpression(
@@ skipping 832 matching lines @@
       // KernelVariableDeclaration.
       mutatedInClosure = true;
       declaredType = variable.type;
     }
     return inferrer.inferVariableGet(typeContext, typeNeeded, mutatedInClosure,
         _fact, _scope, fileOffset, declaredType, (type) {
       promotedType = type;
     });
   }
 }