[flutter_tools] update analyze_once_test.dart to be null-safe (#141790)

Fixes https://github.com/flutter/flutter/issues/141743

I should have made this change in https://github.com/flutter/flutter/pull/124039, but it escaped my grep search.

packages/flutter_tools/test/integration.shard/analyze_once_test.dart

@@ -63,7 +63,7 @@ void main() {
       "name": "flutter_project",
       "rootUri": "../",
       "packageUri": "lib/",
-      "languageVersion": "2.7"
+      "languageVersion": "2.12"
     }
   ]
 }
@@ -170,7 +170,7 @@ void main() {
       statusTextContains: <String>[
         'Analyzing',
         'unused_element',
-        'missing_required_param',
+        'missing_required_argument',
       ],
       exitMessageContains: '2 issues found.',
       exitCode: 1,
@@ -205,7 +205,7 @@ void main() {
         'Analyzing',
         'unused_element',
         'only_throw_errors',
-        'missing_required_param',
+        'missing_required_argument',
       ],
       exitMessageContains: '3 issues found.',
       exitCode: 1,
@@ -266,22 +266,15 @@ StringBuffer bar = StringBuffer('baz');
 
   testWithoutContext('analyze once with default options has info issue finally exit code 1.', () async {
     const String infoSourceCode = '''
-int analyze() {}
+void _analyze() {}
 ''';
 
-    final File optionsFile = fileSystem.file(fileSystem.path.join(projectPath, 'analysis_options.yaml'));
-    optionsFile.writeAsStringSync('''
-analyzer:
-  errors:
-    missing_return: info
-  ''');
-
     fileSystem.directory(projectPath).childFile('main.dart').writeAsStringSync(infoSourceCode);
     await runCommand(
       arguments: <String>['analyze', '--no-pub'],
       statusTextContains: <String>[
         'info',
-        'missing_return',
+        'unused_element',
       ],
       exitMessageContains: '1 issue found.',
       exitCode: 1,
@@ -290,14 +283,14 @@ analyzer:
 
   testWithoutContext('analyze once with no-fatal-infos has info issue finally exit code 0.', () async {
     const String infoSourceCode = '''
-int analyze() {}
+void _analyze() {}
 ''';
 
     final File optionsFile = fileSystem.file(fileSystem.path.join(projectPath, 'analysis_options.yaml'));
     optionsFile.writeAsStringSync('''
 analyzer:
   errors:
-    missing_return: info
+    unused_element: info
   ''');
 
     fileSystem.directory(projectPath).childFile('main.dart').writeAsStringSync(infoSourceCode);
@@ -305,7 +298,7 @@ analyzer:
       arguments: <String>['analyze', '--no-pub', '--no-fatal-infos'],
       statusTextContains: <String>[
         'info',
-        'missing_return',
+        'unused_element',
       ],
       exitMessageContains: '1 issue found.',
     );
@@ -313,14 +306,14 @@ analyzer:
 
   testWithoutContext('analyze once only fatal-warnings has info issue finally exit code 0.', () async {
     const String infoSourceCode = '''
-int analyze() {}
+void _analyze() {}
 ''';
 
     final File optionsFile = fileSystem.file(fileSystem.path.join(projectPath, 'analysis_options.yaml'));
     optionsFile.writeAsStringSync('''
 analyzer:
   errors:
-    missing_return: info
+    unused_element: info
   ''');
 
     fileSystem.directory(projectPath).childFile('main.dart').writeAsStringSync(infoSourceCode);
@@ -328,7 +321,7 @@ analyzer:
       arguments: <String>['analyze', '--no-pub', '--fatal-warnings', '--no-fatal-infos'],
       statusTextContains: <String>[
         'info',
-        'missing_return',
+        'unused_element',
       ],
       exitMessageContains: '1 issue found.',
     );
@@ -336,14 +329,14 @@ analyzer:
 
   testWithoutContext('analyze once only fatal-infos has warning issue finally exit code 0.', () async {
     const String warningSourceCode = '''
-int analyze() {}
+void _analyze() {}
 ''';
 
     final File optionsFile = fileSystem.file(fileSystem.path.join(projectPath, 'analysis_options.yaml'));
     optionsFile.writeAsStringSync('''
 analyzer:
   errors:
-    missing_return: warning
+    unused_element: warning
   ''');
 
     fileSystem.directory(projectPath).childFile('main.dart').writeAsStringSync(warningSourceCode);
@@ -351,7 +344,7 @@ analyzer:
       arguments: <String>['analyze','--no-pub', '--fatal-infos', '--no-fatal-warnings'],
       statusTextContains: <String>[
         'warning',
-        'missing_return',
+        'unused_element',
       ],
       exitMessageContains: '1 issue found.',
     );
@@ -360,14 +353,14 @@ analyzer:
 
   testWithoutContext('analyze once only fatal-warnings has warning issue finally exit code 1.', () async {
     const String warningSourceCode = '''
-int analyze() {}
+void _analyze() {}
 ''';
 
     final File optionsFile = fileSystem.file(fileSystem.path.join(projectPath, 'analysis_options.yaml'));
     optionsFile.writeAsStringSync('''
 analyzer:
   errors:
-    missing_return: warning
+    unused_element: warning
   ''');
 
     fileSystem.directory(projectPath).childFile('main.dart').writeAsStringSync(warningSourceCode);
@@ -375,7 +368,7 @@ analyzer:
       arguments: <String>['analyze','--no-pub', '--no-fatal-infos', '--fatal-warnings'],
       statusTextContains: <String>[
         'warning',
-        'missing_return',
+        'unused_element',
       ],
       exitMessageContains: '1 issue found.',
       exitCode: 1,
@@ -408,7 +401,7 @@ class MyApp extends StatelessWidget {
 }
 
 class MyHomePage extends StatefulWidget {
-  MyHomePage({Key key, this.title}) : super(key: key);
+  MyHomePage({Key? key, required this.title}) : super(key: key);
 
   final String title;