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

packages/image_picker/image_picker_for_web/example/integration_test/image_picker_for_web_test.dart

@@ -13,8 +13,8 @@ import 'package:integration_test/integration_test.dart';
 
 const String expectedStringContents = 'Hello, world!';
 const String otherStringContents = 'Hello again, world!';
-final Uint8List bytes = utf8.encode(expectedStringContents) as Uint8List;
-final Uint8List otherBytes = utf8.encode(otherStringContents) as Uint8List;
+final Uint8List bytes = const Utf8Encoder().convert(expectedStringContents);
+final Uint8List otherBytes = const Utf8Encoder().convert(otherStringContents);
 final Map<String, dynamic> options = <String, dynamic>{
   'type': 'text/plain',
   'lastModified': DateTime.utc(2017, 12, 13).millisecondsSinceEpoch,

packages/rfw/lib/src/dart/binary.dart

@@ -521,7 +521,7 @@ class _BlobEncoder {
   }
 
   void _writeString(String value) {
-    final Uint8List buffer = utf8.encode(value) as Uint8List;
+    final Uint8List buffer = const Utf8Encoder().convert(value);
     _writeInt64(buffer.length);
     bytes.add(buffer);
   }