make it compile on dart2.12/flutter2.0

flutter/test/default_integrations_test.dart

@@ -106,7 +106,7 @@ void main() {
 
   test('nativeSdkIntegration do not throw', () async {
     _channel.setMockMethodCallHandler((MethodCall methodCall) async {
-      throw null;
+      throw Exception();
     });
 
     final integration = NativeSdkIntegration(_channel);

flutter/test/file_system_transport_test.dart

@@ -33,7 +33,7 @@ void main() {
 
   test('FileSystemTransport returns emptyId if channel throws', () async {
     _channel.setMockMethodCallHandler((MethodCall methodCall) async {
-      throw null;
+      throw Exception();
     });
 
     final transport = fixture.getSut(_channel);

flutter/test/load_android_image_list_test.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:sentry/sentry.dart';

flutter/test/load_contexts_integrations_test.dart

@@ -1,3 +1,5 @@
+import 'dart:async';
+
 import 'package:flutter/services.dart';
 import 'package:flutter_test/flutter_test.dart';
 import 'package:sentry_flutter/sentry_flutter.dart';
@@ -128,7 +130,7 @@ void main() {
 
   test('should not throw on loadContextsIntegration exception', () async {
     _channel.setMockMethodCallHandler((MethodCall methodCall) async {
-      throw null;
+      throw Exception();
     });
     final options = SentryFlutterOptions()..dsn = fakeDsn;
     final hub = Hub(options);