In strong mode, properly infer the return types of setters as `void`

pkg/analyzer/test/src/task/strong/front_end_inference_test.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.
 
 import 'dart:async';
 import 'dart:convert';
 import 'dart:io';
 
 import 'package:analyzer/dart/ast/ast.dart';
 import 'package:analyzer/dart/ast/token.dart';
@@ skipping 439 matching lines @@
         _recordTypeArguments(node.offset, type.typeArguments);
       }
     }
   }
 
   @override
   visitMethodDeclaration(MethodDeclaration node) {
     super.visitMethodDeclaration(node);
     if (node.element.enclosingElement is ClassElement && !node.isStatic) {
       if (node.returnType == null) {
-        // For now, we skip inferred return types of setters.
-        // TODO(paulberry): fix this.
-        if (!node.isSetter) {
-          _recordTopType(node.name.offset, node.element.returnType);
-        }
+        _recordTopType(node.name.offset, node.element.returnType);
       }
       if (node.parameters != null) {
         for (var parameter in node.parameters.parameters) {
           // Note: it's tempting to check `parameter.type == null`, but that
           // doesn't work because of function-typed formal parameter syntax.
           if (parameter.element.hasImplicitType) {
             _recordTopType(parameter.identifier.offset, parameter.element.type);
           }
         }
       }
@@ skipping 112 matching lines @@
 
   /// Based on DDC code generator's `_recoverTypeArguments`
   Iterable<DartType> _recoverTypeArguments(FunctionType g, FunctionType f) {
     assert(identical(g.element, f.element));
     assert(g.typeFormals.isNotEmpty && f.typeFormals.isEmpty);
     assert(g.typeFormals.length + g.typeArguments.length ==
         f.typeArguments.length);
     return f.typeArguments.skip(g.typeArguments.length);
   }
 }