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

pkg/analyzer/test/generated/all_the_rest_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.all_the_rest_test;
 
 import 'package:analyzer/dart/element/element.dart';
 import 'package:analyzer/dart/element/type.dart';
 import 'package:analyzer/file_system/physical_file_system.dart';
 import 'package:analyzer/src/dart/element/element.dart';
@@ skipping 5317 matching lines @@
             AstFactory.formalParameterList(), AstFactory.blockFunctionBody2()));
     declaration.documentationComment = AstFactory.documentationComment(
         [TokenFactory.tokenFromString('/// aaa')..offset = 50], []);
     declaration.accept(builder);
 
     List<PropertyAccessorElement> accessors = holder.accessors;
     expect(accessors, hasLength(1));
     PropertyAccessorElement accessor = accessors[0];
     expect(accessor, isNotNull);
     _assertHasDocRange(accessor, 50, 7);
-    expect(accessor.hasImplicitReturnType, isFalse);
+    expect(accessor.hasImplicitReturnType, isTrue);
     expect(accessor.name, "$functionName=");
     expect(declaration.element, same(accessor));
     expect(declaration.functionExpression.element, same(accessor));
     expect(accessor.isGetter, isFalse);
     expect(accessor.isExternal, isFalse);
     expect(accessor.isSetter, isTrue);
     expect(accessor.isSynthetic, isFalse);
     expect(accessor.typeParameters, hasLength(0));
     PropertyInducingElement variable = accessor.variable;
     EngineTestCase.assertInstanceOf((obj) => obj is TopLevelVariableElement,
@@ skipping 406 matching lines @@
     expect(fields, hasLength(1));
     FieldElement field = fields[0];
     expect(field, isNotNull);
     expect(field.name, methodName);
     expect(field.isSynthetic, isTrue);
     expect(field.getter, isNull);
 
     PropertyAccessorElement setter = field.setter;
     expect(setter, isNotNull);
     _assertHasDocRange(setter, 50, 7);
-    expect(setter.hasImplicitReturnType, isFalse);
+    expect(setter.hasImplicitReturnType, isTrue);
     expect(setter.isAbstract, isFalse);
     expect(setter.isExternal, isFalse);
     expect(setter.isSetter, isTrue);
     expect(setter.isSynthetic, isFalse);
     expect(setter.name, "$methodName=");
     expect(setter.displayName, methodName);
     expect(setter.variable, field);
     expect(setter.functions, hasLength(0));
     expect(setter.labels, hasLength(0));
     expect(setter.localVariables, hasLength(0));
@@ skipping 17 matching lines @@
 
     List<FieldElement> fields = holder.fields;
     expect(fields, hasLength(1));
     FieldElement field = fields[0];
     expect(field, isNotNull);
     expect(field.name, methodName);
     expect(field.isSynthetic, isTrue);
     expect(field.getter, isNull);
     PropertyAccessorElement setter = field.setter;
     expect(setter, isNotNull);
-    expect(setter.hasImplicitReturnType, isFalse);
+    expect(setter.hasImplicitReturnType, isTrue);
     expect(setter.isAbstract, isTrue);
     expect(setter.isExternal, isFalse);
     expect(setter.isSetter, isTrue);
     expect(setter.isSynthetic, isFalse);
     expect(setter.name, "$methodName=");
     expect(setter.displayName, methodName);
     expect(setter.variable, field);
     expect(setter.functions, hasLength(0));
     expect(setter.labels, hasLength(0));
     expect(setter.localVariables, hasLength(0));
@@ skipping 18 matching lines @@
 
     List<FieldElement> fields = holder.fields;
     expect(fields, hasLength(1));
     FieldElement field = fields[0];
     expect(field, isNotNull);
     expect(field.name, methodName);
     expect(field.isSynthetic, isTrue);
     expect(field.getter, isNull);
     PropertyAccessorElement setter = field.setter;
     expect(setter, isNotNull);
-    expect(setter.hasImplicitReturnType, isFalse);
+    expect(setter.hasImplicitReturnType, isTrue);
     expect(setter.isAbstract, isFalse);
     expect(setter.isExternal, isTrue);
     expect(setter.isSetter, isTrue);
     expect(setter.isSynthetic, isFalse);
     expect(setter.name, "$methodName=");
     expect(setter.displayName, methodName);
     expect(setter.variable, field);
     expect(setter.functions, hasLength(0));
     expect(setter.labels, hasLength(0));
     expect(setter.localVariables, hasLength(0));
@@ skipping 2391 matching lines @@
     expect(UriKind.fromEncoding(0x70), same(UriKind.PACKAGE_URI));
     expect(UriKind.fromEncoding(0x58), same(null));
   }
 
   void test_getEncoding() {
     expect(UriKind.DART_URI.encoding, 0x64);
     expect(UriKind.FILE_URI.encoding, 0x66);
     expect(UriKind.PACKAGE_URI.encoding, 0x70);
   }
 }