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

pkg/analyzer/test/src/summary/summary_test.dart

 // Copyright (c) 2015, 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.src.summary.summary_test;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/src/generated/ast.dart';
 import 'package:analyzer/src/generated/engine.dart';
 import 'package:analyzer/src/generated/error.dart';
@@ skipping 1376 matching lines @@
     UnlinkedExecutable executable =
         findExecutable('f', executables: cls.executables, failIfAbsent: true);
     expect(executable.isExternal, isTrue);
   }
 
   test_executable_member_setter() {
     UnlinkedClass cls = serializeClassText('class C { void set f(value) {} }');
     UnlinkedExecutable executable =
         findExecutable('f=', executables: cls.executables, failIfAbsent: true);
     expect(executable.kind, UnlinkedExecutableKind.setter);
-    // For setters, hasImplicitReturnType is always false.
     expect(executable.hasImplicitReturnType, isFalse);
     expect(executable.isExternal, isFalse);
     expect(findVariable('f', variables: cls.fields), isNull);
     expect(findExecutable('f', executables: cls.executables), isNull);
   }
 
   test_executable_member_setter_external() {
     UnlinkedClass cls =
         serializeClassText('class C { external void set f(value); }');
     UnlinkedExecutable executable =
         findExecutable('f=', executables: cls.executables, failIfAbsent: true);
     expect(executable.isExternal, isTrue);
   }
 
   test_executable_member_setter_implicit_return() {
     UnlinkedClass cls = serializeClassText('class C { set f(value) {} }');
     UnlinkedExecutable executable =
         findExecutable('f=', executables: cls.executables, failIfAbsent: true);
-    expect(executable.hasImplicitReturnType, isFalse);
+    expect(executable.hasImplicitReturnType, isTrue);
     checkDynamicTypeRef(executable.returnType);
   }
 
   test_executable_name() {
     UnlinkedExecutable executable = serializeExecutableText('f() {}');
     expect(executable.name, 'f');
   }
 
   test_executable_no_flags() {
     UnlinkedExecutable executable = serializeExecutableText('f() {}');
@@ skipping 228 matching lines @@
 
   test_executable_setter_external() {
     UnlinkedExecutable executable =
         serializeExecutableText('external void set f(value);', 'f=');
     expect(executable.isExternal, isTrue);
   }
 
   test_executable_setter_implicit_return() {
     UnlinkedExecutable executable =
         serializeExecutableText('set f(value) {}', 'f=');
-    // For setters, hasImplicitReturnType is always false.
-    expect(executable.hasImplicitReturnType, isFalse);
+    expect(executable.hasImplicitReturnType, isTrue);
     checkDynamicTypeRef(executable.returnType);
   }
 
   test_executable_setter_private() {
     serializeExecutableText('void set _f(value) {}', '_f=');
     expect(unlinkedUnits[0].publicNamespace.names, isEmpty);
   }
 
   test_executable_setter_type() {
     UnlinkedExecutable executable =
@@ skipping 686 matching lines @@
     UnlinkedVariable variable =
         serializeVariableText('int i;', variableName: 'i');
     checkTypeRef(variable.type, 'dart:core', 'dart:core', 'int');
   }
 
   test_varible_private() {
     serializeVariableText('int _i;', variableName: '_i');
     expect(unlinkedUnits[0].publicNamespace.names, isEmpty);
   }
 }