MIgrate dev_integration_tests_web to null safety (#80625)

dev/integration_tests/web/lib/framework_stack_trace.dart

@@ -17,7 +17,7 @@ import 'package:flutter/widgets.dart';
 // framework's ability to parse stack traces in all build modes.
 void main() async {
   final StringBuffer errorMessage = StringBuffer();
-  debugPrint = (String message, { int wrapWidth }) {
+  debugPrint = (String? message, { int? wrapWidth }) {
     errorMessage.writeln(message);
   };
 

dev/integration_tests/web/lib/service_worker_test.dart

@@ -4,8 +4,8 @@
 
 import 'dart:html' as html;
 Future<void> main() async {
-  await html.window.navigator.serviceWorker.ready;
+  await html.window.navigator.serviceWorker?.ready;
   final String response = 'CLOSE?version=1';
   await html.HttpRequest.getString(response);
-  html.document.body.appendHtml(response);
+  html.document.body?.appendHtml(response);
 }

dev/integration_tests/web/lib/stack_trace.dart

@@ -158,5 +158,5 @@ class StackFrameEquality implements Equality<StackFrame> {
   }
 
   @override
-  bool isValidKey(Object o) => o is StackFrame;
+  bool isValidKey(Object? o) => o is StackFrame;
 }

dev/integration_tests/web/lib/web_directory_loading.dart

@@ -11,7 +11,7 @@ Future<void> main() async {
       '/example',
       method: 'GET',
     );
-    final String body = request.responseText;
+    final String? body = request.responseText;
     if (body == 'This is an Example') {
       print('--- TEST SUCCEEDED ---');
     } else {

dev/integration_tests/web/pubspec.yaml

@@ -2,7 +2,7 @@ name: web_integration
 description: Integration test for web compilation.
 
 environment:
-  sdk: ">=2.6.0 <3.0.0"
+  sdk: ">=2.12.0 <3.0.0"
 
 flutter:
   assets: