Prepare for utf8.encode() to return more precise Uint8List type (#43335)

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

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);
 }