Infer return types of closures (basic support).

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 499 matching lines @@
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     // TODO(scheglov): implement.
     return typeNeeded ? const DynamicType() : null;
   }
 }
 
 /// Concrete shadow object representing a return statement in kernel form.
 class KernelReturnStatement extends ReturnStatement implements KernelStatement {
-  KernelReturnStatement([KernelExpression expression]) : super(expression);
+  KernelReturnStatement([Expression expression]) : super(expression);
 
   @override
   void _inferStatement(KernelTypeInferrer inferrer) {
-    // TODO(paulberry): implement.
+    inferrer.inferReturnStatement(expression);
   }
 }
 
 /// Common base class for shadow objects representing statements in kernel
 /// form.
 abstract class KernelStatement extends Statement {
   /// Calls back to [inferrer] to perform type inference for whatever concrete
   /// type of [KernelStatement] this is.
   void _inferStatement(KernelTypeInferrer inferrer);
 }
@@ skipping 453 matching lines @@
       : super(variable, value);
 
   @override
   DartType _inferExpression(
       KernelTypeInferrer inferrer, DartType typeContext, bool typeNeeded) {
     var variable = this.variable as KernelVariableDeclaration;
     return inferrer.inferVariableSet(
         typeContext, typeNeeded, variable._declaredType, value);
   }
 }