migrate fixtures/_test to null safety

diff --git a/fixtures/_test/example/append_body/main.dart b/fixtures/_test/example/append_body/main.dart
index b35e191..6671c46 100644
--- a/fixtures/_test/example/append_body/main.dart
+++ b/fixtures/_test/example/append_body/main.dart

@@ -13,12 +13,12 @@
     print('Count is: ${++count}'); // Breakpoint: printCount
   });
 
-  document.body.appendText('Hello World!');
+  document.body!.appendText('Hello World!');
 
   registerExtension('ext.flutter.disassemble', (_, __) async {
-    document.body.appendText('start disassemble ');
+    document.body!.appendText('start disassemble ');
     await Future.delayed(const Duration(seconds: 1));
-    document.body.appendText('end disassemble ');
+    document.body!.appendText('end disassemble ');
     return ServiceExtensionResponse.result('{}');
   });
 }

diff --git a/fixtures/_test/example/hello_world/main.dart b/fixtures/_test/example/hello_world/main.dart
index f36903c..3a63860 100644
--- a/fixtures/_test/example/hello_world/main.dart
+++ b/fixtures/_test/example/hello_world/main.dart

@@ -39,7 +39,7 @@
   context['registerExtension'] = (String method) {
     registerExtension(method,
         (String method, Map<String, String> parameters) async {
-      return ServiceExtensionResponse.result(jsonEncode(parameters ?? {}));
+      return ServiceExtensionResponse.result(jsonEncode(parameters));
     });
   };
 
@@ -47,7 +47,7 @@
     registerExtension(method,
         (String method, Map<String, String> parameters) async {
       return ServiceExtensionResponse.error(
-          int.parse(parameters['code']), parameters['details']);
+          int.parse(parameters['code']!), parameters['details']!);
     });
   };
 
@@ -73,7 +73,7 @@
 
   // Register one up front before the proxy connects, the isolate should still
   // recognize this as an available extension.
-  registerExtension('ext.hello_world.existing', (_, __) => null);
+  registerExtension('ext.hello_world.existing', (_, __) async => ServiceExtensionResponse.result('dummy'));
 
   window.console.debug('Page Ready');
 }
@@ -119,7 +119,7 @@
 class MyTestClass {
   final String message;
 
-  String notFinal;
+  String? notFinal;
 
   static final String staticMessage = 'static';
 

diff --git a/fixtures/_test/pubspec.yaml b/fixtures/_test/pubspec.yaml
index 8bb81a5..0147fe2 100644
--- a/fixtures/_test/pubspec.yaml
+++ b/fixtures/_test/pubspec.yaml

@@ -5,12 +5,12 @@
 publish_to: none
 
 environment:
-  sdk: ">=2.5.0 <3.0.0"
+  sdk: '>=2.12.0 <3.0.0'
 
 dependencies:
-  intl: ^0.16.0
-  path: ^1.6.1
+  intl: ^0.17.0
+  path: ^1.8.2
 
 dev_dependencies:
-  build_runner: ^2.0.0
-  build_web_compilers: ^3.0.0
+  build_runner: ^2.1.11
+  build_web_compilers: ^3.2.3