fix #25280, treat setters as returning void in strong mode

pkg/analyzer/test/generated/resolver_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.resolver_test;
 
 import 'dart:collection';
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
@@ skipping 13492 matching lines @@
     _resolveTestUnit(code);
 
     SimpleIdentifier identifier = _findIdentifier('foo');
     VariableDeclaration declaration =
         identifier.getAncestor((node) => node is VariableDeclaration);
 
     expect(declaration.initializer.staticType.toString(), "Future<String>");
     expect(declaration.initializer.propagatedType, isNull);
   }
 
+  void test_setterWithDynamicTypeIsError() {
+    Source source = addSource(r'''
+class A {
+  dynamic set f(String s) => null;
+}
+dynamic set g(int x) => null;
+''');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [
+      StaticWarningCode.NON_VOID_RETURN_FOR_SETTER,
+      StaticWarningCode.NON_VOID_RETURN_FOR_SETTER
+    ]);
+    verify([source]);
+  }
+
+  void test_setterWithExplicitVoidType_returningVoid() {
+    Source source = addSource(r'''
+void returnsVoid() {}
+class A {
+  void set f(String s) => returnsVoid();
+}
+void set g(int x) => returnsVoid();
+''');
+    computeLibrarySourceErrors(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
+  void test_setterWithNoVoidType() {
+    Source source = addSource(r'''
+class A {
+  set f(String s) {
+    return '42';
+  }
+}
+set g(int x) => 42;
+''');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [
+      StaticTypeWarningCode.RETURN_OF_INVALID_TYPE,
+      StaticTypeWarningCode.RETURN_OF_INVALID_TYPE
+    ]);
+    verify([source]);
+  }
+
+  void test_setterWithNoVoidType_returningVoid() {
+    Source source = addSource(r'''
+void returnsVoid() {}
+class A {
+  set f(String s) => returnsVoid();
+}
+set g(int x) => returnsVoid();
+''');
+    computeLibrarySourceErrors(source);
+    assertNoErrors(source);
+    verify([source]);
+  }
+
+  void test_setterWithOtherTypeIsError() {
+    Source source = addSource(r'''
+class A {
+  String set f(String s) => null;
+}
+Object set g(x) => null;
+''');
+    computeLibrarySourceErrors(source);
+    assertErrors(source, [
+      StaticWarningCode.NON_VOID_RETURN_FOR_SETTER,
+      StaticWarningCode.NON_VOID_RETURN_FOR_SETTER
+    ]);
+    verify([source]);
+  }
+
   void test_ternaryOperator_null_left() {
     String code = r'''
 main() {
   var foo = (true) ? null : 3;
 }
 ''';
     _resolveTestUnit(code);
 
     SimpleIdentifier identifier = _findIdentifier('foo');
     VariableDeclaration declaration =
@@ skipping 2856 matching lines @@
 
   void _resolveTestUnit(String code) {
     testCode = code;
     testSource = addSource(testCode);
     LibraryElement library = resolve2(testSource);
     assertNoErrors(testSource);
     verify([testSource]);
     testUnit = resolveCompilationUnit(testSource, library);
   }
 }