analyzer_testing: Widget is @immutable

This requires some changes to tests, where fields should be final. In
tests where it seemed like the mutability of a field was important, I
just ignored the warning. Otherwise, I made fields final.

I do like keeping this stub in-line with the real Widget class. If
anything, it keeps in mind that flutter Widgets should generally no
have mutable fields.

Change-Id: Ib7dc1fbd6aa013dc72925404c4be8186ddcddaa4
Reviewed-on: https://dart-review.googlesource.com/c/sdk/+/475400
Reviewed-by: Brian Wilkerson <brianwilkerson@google.com>
Commit-Queue: Samuel Rawlins <srawlins@google.com>

Author: srawlins

pkg/analysis_server/test/services/refactoring/legacy/extract_widget_test.dart

@@ -371,7 +371,7 @@ class C {
 }
 
 class MyWidget extends StatelessWidget {
-  C c = C();
+  final C c = C();
 
   @override
   Widget build(BuildContext context) {
@@ -394,7 +394,7 @@ class C {
 }
 
 class MyWidget extends StatelessWidget {
-  C c = C();
+  final C c = C();
 
   @override
   Widget build(BuildContext context) {
@@ -476,7 +476,7 @@ class Test extends StatelessWidget {
 import 'package:flutter/material.dart';
 
 class MyWidget extends StatelessWidget {
-  String foo = '';
+  final String foo = '';
 
   @override
   Widget build(BuildContext context) {
@@ -504,7 +504,7 @@ class MyWidget extends StatelessWidget {
 import 'package:flutter/material.dart';
 
 class MyWidget extends StatelessWidget {
-  String foo = '';
+  final String foo = '';
 
   @override
   Widget build(BuildContext context) {
@@ -720,7 +720,7 @@ class C {
 }
 
 class MyWidget extends StatelessWidget {
-  C c = C();
+  final C c = C();
 
   @override
   Widget build(BuildContext context) {
@@ -738,7 +738,7 @@ class C {
 }
 
 class MyWidget extends StatelessWidget {
-  C c = C();
+  final C c = C();
 
   @override
   Widget build(BuildContext context) {
@@ -806,6 +806,7 @@ class Test extends StatelessWidget {
     await indexTestUnit(r'''
 import 'package:flutter/material.dart';
 
+// ignore: must_be_immutable
 class MyWidget extends StatelessWidget {
   String field;
 
@@ -832,10 +833,12 @@ class MyWidget extends StatelessWidget {
     await indexTestUnit(r'''
 import 'package:flutter/material.dart';
 
+// ignore: must_be_immutable
 abstract class MySuperWidget extends StatelessWidget {
   String field = '';
 }
 
+// ignore: must_be_immutable
 class MyWidget extends MySuperWidget {
   @override
   Widget build(BuildContext context) {
@@ -863,7 +866,7 @@ class C {
 }
 
 class MyWidget extends StatelessWidget {
-  C c = C();
+  final C c = C();
 
   @override
   Widget build(BuildContext context) {
@@ -886,7 +889,7 @@ class C {
 }
 
 class MyWidget extends StatelessWidget {
-  C c = C();
+  final C c = C();
 
   @override
   Widget build(BuildContext context) {

pkg/analysis_server/test/src/services/correction/assist/add_diagnostic_property_reference_test.dart

@@ -32,7 +32,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 class W extends Widget {
-  bool ^property = true;
+  final bool ^property = true;
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);
@@ -44,7 +44,7 @@ import 'package:flutter/foundation.dart';
 import 'package:flutter/widgets.dart';
 
 class W extends Widget {
-  bool property = true;
+  final bool property = true;
   @override
   void debugFillProperties(DiagnosticPropertiesBuilder properties) {
     super.debugFillProperties(properties);

pkg/analysis_server/test/src/services/correction/assist/flutter_convert_to_stateful_widget_test.dart

@@ -175,6 +175,7 @@ class _MyWidgetState<T> extends State<MyWidget<T>> {
     await resolveTestCode(r'''
 import 'package:flutter/material.dart';
 
+// ignore: must_be_immutable
 class ^MyWidget extends StatelessWidget {
   static String staticField1 = '';
   final String instanceField1;
@@ -210,6 +211,7 @@ class ^MyWidget extends StatelessWidget {
     await assertHasAssist(r'''
 import 'package:flutter/material.dart';
 
+// ignore: must_be_immutable
 class MyWidget extends StatefulWidget {
   static String staticField1 = '';
   final String instanceField1;
@@ -316,7 +318,7 @@ import 'package:flutter/material.dart';
 class ^MyWidget extends StatelessWidget {
   static String staticField = '';
   final String instanceField1;
-  String instanceField2 = '';
+  final String instanceField2 = '';
 
   MyWidget(this.instanceField1);
 
@@ -372,7 +374,7 @@ class MyWidget extends StatefulWidget {
 }
 
 class _MyWidgetState extends State<MyWidget> {
-  String instanceField2 = '';
+  final String instanceField2 = '';
 
   @override
   Widget build(BuildContext context) {

pkg/analysis_server/test/src/services/correction/assist/flutter_convert_to_stateless_widget_test.dart

@@ -157,6 +157,7 @@ class MyWidget extends StatelessWidget {
     await resolveTestCode(r'''
 import 'package:flutter/material.dart';
 
+// ignore: must_be_immutable
 class ^MyWidget extends StatefulWidget {
   static String staticField1 = '';
   final String instanceField1;
@@ -203,6 +204,7 @@ class _MyWidgetState extends State<MyWidget> {
     await assertHasAssist(r'''
 import 'package:flutter/material.dart';
 
+// ignore: must_be_immutable
 class MyWidget extends StatelessWidget {
   static String staticField1 = '';
   final String instanceField1;

pkg/analysis_server/test/src/services/correction/assist/flutter_wrap_generic_test.dart

@@ -203,7 +203,7 @@ class FakeFlutter {\r
 import 'package:flutter/widgets.dart';
 
 abstract class Foo extends Widget {
-  Widget bar = Text('');
+  final Widget bar = Text('');
 }
 
 Widget f(Foo foo) {
@@ -214,7 +214,7 @@ Widget f(Foo foo) {
 import 'package:flutter/widgets.dart';
 
 abstract class Foo extends Widget {
-  Widget bar = Text('');
+  final Widget bar = Text('');
 }
 
 Widget f(Foo foo) {
@@ -228,7 +228,7 @@ Widget f(Foo foo) {
 import 'package:flutter/widgets.dart';
 
 abstract class Foo extends Widget {
-  Widget bar = Text('');
+  final Widget bar = Text('');
 }
 
 Widget f(Foo foo) {
@@ -239,7 +239,7 @@ Widget f(Foo foo) {
 import 'package:flutter/widgets.dart';
 
 abstract class Foo extends Widget {
-  Widget bar = Text('');
+  final Widget bar = Text('');
 }
 
 Widget f(Foo foo) {