Restore weak mode errors on empty returns in Null functions

pkg/analyzer/test/generated/static_warning_code_test.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.test.generated.static_warning_code_test;
 
 import 'package:analyzer/error/error.dart';
 import 'package:analyzer/src/error/codes.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/source_io.dart';
@@ skipping 3044 matching lines @@
   test_redirectToNonClass_undefinedIdentifier() async {
     Source source = addSource(r'''
 class B {
   factory B() = A;
 }''');
     await computeAnalysisResult(source);
     assertErrors(source, [StaticWarningCode.REDIRECT_TO_NON_CLASS]);
     verify([source]);
   }
 
+  test_returnWithoutValue_Null() async {
+    // Test that block bodied functions with return type Null and an empty
+    // return cause a static warning.
+    Source source = addSource(r'''
+Null f() {return;}
+''');
+    await computeAnalysisResult(source);
+    assertErrors(source, [StaticWarningCode.RETURN_WITHOUT_VALUE]);
+  }
+
   test_returnWithoutValue_async() async {
     Source source = addSource('''
 import 'dart:async';
 Future<int> f() async {
   return;
 }
 ''');
     await computeAnalysisResult(source);
     assertErrors(source, [StaticWarningCode.RETURN_WITHOUT_VALUE]);
     verify([source]);
@@ skipping 642 matching lines @@
 
   test_voidReturnForGetter() async {
     Source source = addSource(r'''
 class S {
   void get value {}
 }''');
     await computeAnalysisResult(source);
     assertErrors(source, [StaticWarningCode.VOID_RETURN_FOR_GETTER]);
   }
 }