Allow values to be returned from implicitly typed setters

pkg/analyzer/lib/src/generated/error_verifier.dart

 // Copyright (c) 2014, 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.
 
 library analyzer.src.generated.error_verifier;
 
 import 'dart:collection';
 import "dart:math" as math;
 
 import 'package:analyzer/dart/ast/ast.dart';
@@ skipping 624 matching lines @@
     bool wasInAsync = _inAsync;
     bool wasInGenerator = _inGenerator;
     try {
       _inAsync = node.isAsynchronous;
       _inGenerator = node.isGenerator;
       FunctionType functionType =
           _enclosingFunction == null ? null : _enclosingFunction.type;
       DartType expectedReturnType = functionType == null
           ? DynamicTypeImpl.instance
           : functionType.returnType;
-      _checkForReturnOfInvalidType(node.expression, expectedReturnType);
+      bool isSetterWithImplicitReturn =
+          _enclosingFunction.hasImplicitReturnType &&
+          _enclosingFunction is PropertyAccessorElement &&
+          (_enclosingFunction as PropertyAccessorElement).isSetter;
+      if (!isSetterWithImplicitReturn) {
+        _checkForReturnOfInvalidType(node.expression, expectedReturnType);
+      }
       return super.visitExpressionFunctionBody(node);
     } finally {
       _inAsync = wasInAsync;
       _inGenerator = wasInGenerator;
     }
   }
 
   @override
   Object visitFieldDeclaration(FieldDeclaration node) {
     _isInStaticVariableDeclaration = node.isStatic;
@@ skipping 5402 matching lines @@
 class _InvocationCollector extends RecursiveAstVisitor {
   final List<String> superCalls = <String>[];
 
   @override
   visitMethodInvocation(MethodInvocation node) {
     if (node.target is SuperExpression) {
       superCalls.add(node.methodName.name);
     }
   }
 }