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

pkg/analyzer/test/generated/resolver_test.dart

Index: pkg/analyzer/test/generated/resolver_test.dart
diff --git a/pkg/analyzer/test/generated/resolver_test.dart b/pkg/analyzer/test/generated/resolver_test.dart
index fa4338d94762abdcea34eb0a568bf1fa1ddbd028..7e965fad74be8800b9ad2655d6842dc25944be9d 100644
--- a/pkg/analyzer/test/generated/resolver_test.dart
+++ b/pkg/analyzer/test/generated/resolver_test.dart
@@ -13220,6 +13220,36 @@ main() {
     expect(declaration.initializer.propagatedType, isNull);
   }
 
+  void test_setterWithNoVoidType() {

[Brian Wilkerson, 2015/12/17 17:58:00]

In addition to the explicit 'dynamic' case, we should also test an explicit
non-dynamic case.

[Jennifer Messerly, 2016/01/05 00:42:44]

Good catch. Done & done.

+    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_ternaryOperator_null_left() {
     String code = r'''
 main() {