Remove unneccessary casts. (#2930)

*Replace this paragraph with a description of what this PR is changing or adding, and why. Consider including before/after screenshots.*

*List which issues are fixed by this PR. You must list at least one issue.*

*If you had to change anything in the [flutter/tests] repo, include a link to the migration guide as per the [breaking change policy].*

app_dart/pubspec.yaml

@@ -8,7 +8,7 @@ homepage: https://github.com/flutter/cocoon
 publish_to: none
 
 environment:
-  sdk: '>=2.19.0-0 <4.0.0'
+  sdk: '>=3.0.0-0 <4.0.0'
 
 dependencies:
   appengine: 0.13.5

app_dart/test/request_handlers/github_webhook_test.dart

@@ -66,8 +66,8 @@ void main() {
   test('Publishes message', () async {
     request.headers.set('X-GitHub-Event', 'pull_request');
     request.body = '{}';
-    final Uint8List body = utf8.encode(request.body!) as Uint8List;
-    final Uint8List key = utf8.encode(keyString) as Uint8List;
+    final Uint8List body = utf8.encode(request.body!);
+    final Uint8List key = utf8.encode(keyString);
     final String hmac = getHmac(body, key);
     request.headers.set('X-Hub-Signature', 'sha1=$hmac');
     await tester.post(webhook);

app_dart/test/request_handlers/query_github_graphql_test.dart

@@ -3,7 +3,6 @@
 // found in the LICENSE file.
 
 import 'dart:convert';
-import 'dart:typed_data';
 
 import 'package:cocoon_service/src/request_handlers/query_github_graphql.dart';
 import 'package:cocoon_service/src/request_handling/body.dart';
@@ -38,15 +37,15 @@ void main() {
       handler = QueryGithubGraphql(
         config: config,
         authenticationProvider: FakeAuthenticationProvider(),
-        requestBodyValue: utf8.encode(graphQLHelloWorld) as Uint8List,
+        requestBodyValue: utf8.encode(graphQLHelloWorld),
       );
     });
 
     test('Empty body raises a bad request exception', () async {
       handler = QueryGithubGraphql(
         config: config,
         authenticationProvider: FakeAuthenticationProvider(),
-        requestBodyValue: utf8.encode('') as Uint8List,
+        requestBodyValue: utf8.encode(''),
       );
       expect(() => tester.post(handler), throwsA(isA<BadRequestException>()));
     });

app_dart/test/src/request_handling/fake_http.dart

@@ -21,8 +21,8 @@ class _Body {
 
   _Body.utf8(String this.value)
       : isUtf8 = true,
-        bytes = utf8.encode(value) as Uint8List,
-        stream = Stream<Uint8List>.fromIterable(<Uint8List>[utf8.encode(value) as Uint8List]);
+        bytes = utf8.encode(value),
+        stream = Stream<Uint8List>.fromIterable(<Uint8List>[utf8.encode(value)]);
 
   _Body.rawBytes(this.bytes)
       : isUtf8 = false,

auto_submit/test/requests/github_webhook_test.dart

@@ -36,8 +36,8 @@ void main() {
     });
 
     test('call handler to handle the post request', () async {
-      final Uint8List body = utf8.encode(generateWebhookEvent()) as Uint8List;
-      final Uint8List key = utf8.encode(keyString) as Uint8List;
+      final Uint8List body = utf8.encode(generateWebhookEvent());
+      final Uint8List key = utf8.encode(keyString);
       final String hmac = getHmac(body, key);
       validHeader = <String, String>{'X-Hub-Signature': 'sha1=$hmac', 'X-GitHub-Event': 'yes'};
       req = Request('POST', Uri.parse('http://localhost/'), body: generateWebhookEvent(), headers: validHeader);