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

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

Index: pkg/analyzer/test/src/summary/summary_test.dart
diff --git a/pkg/analyzer/test/src/summary/summary_test.dart b/pkg/analyzer/test/src/summary/summary_test.dart
index 959526e34975a2f30d1367a9f62f8e1fb50b1e5f..9e2c4ac503ec6fd1e80cab2a70107002ca3ac99d 100644
--- a/pkg/analyzer/test/src/summary/summary_test.dart
+++ b/pkg/analyzer/test/src/summary/summary_test.dart
@@ -1394,7 +1394,6 @@ typedef F();
     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);
@@ -1413,7 +1412,7 @@ typedef F();
     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);
   }
 
@@ -1662,8 +1661,7 @@ typedef F();
   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);
   }