Prepare for utf8.decode() to return more precise Uint8List type

To avoid analyzer warnings when utf8.encode() will return the more precise Uint8List type, we use const Utf8Encoder().convert() which already returns Uint8List

See dart-lang/sdk#52801

Author: mkustermann

lib/web_ui/test/canvaskit/fragment_program_test.dart

@@ -186,7 +186,7 @@ void testMain() {
   });
 
   test('FragmentProgram can be created from JSON IPLR bundle', () {
-    final Uint8List data = utf8.encode(kJsonIPLR) as Uint8List;
+    final Uint8List data = const Utf8Encoder().convert(kJsonIPLR);
     final CkFragmentProgram program = CkFragmentProgram.fromBytes('test', data);
 
     expect(program.effect, isNotNull);

lib/web_ui/test/engine/channel_buffers_test.dart

@@ -23,7 +23,7 @@ void main() {
 }
 
 ByteData _makeByteData(String str) {
-  final Uint8List list = utf8.encode(str) as Uint8List;
+  final Uint8List list = const Utf8Encoder().convert(str);
   final ByteBuffer buffer = list.buffer;
   return ByteData.view(buffer);
 }

testing/dart/channel_buffers_test.dart

@@ -12,7 +12,7 @@ import 'dart:ui' as ui;
 import 'package:litetest/litetest.dart';
 
 ByteData _makeByteData(String str) {
-  final Uint8List list = utf8.encode(str) as Uint8List;
+  final Uint8List list = const Utf8Encoder().convert(str);
   final ByteBuffer buffer = list.buffer;
   return ByteData.view(buffer);
 }