Restore weak mode errors on empty returns in Null functions

pkg/analyzer/test/generated/static_warning_code_test.dart

Index: pkg/analyzer/test/generated/static_warning_code_test.dart
diff --git a/pkg/analyzer/test/generated/static_warning_code_test.dart b/pkg/analyzer/test/generated/static_warning_code_test.dart
index c196f21897e7e4aed6b5a712e816bed2a54622a3..c808b49ec12e0c75d4f42e1d73e1f764bdf345e6 100644
--- a/pkg/analyzer/test/generated/static_warning_code_test.dart
+++ b/pkg/analyzer/test/generated/static_warning_code_test.dart
@@ -3062,6 +3062,16 @@ class B {
     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';