Remove deprecated string features.

pkg/http/test/client_test.dart

Index: pkg/http/test/client_test.dart
diff --git a/pkg/http/test/client_test.dart b/pkg/http/test/client_test.dart
index fd056c9c4233b18b8f8571bff339d73751ebe9ad..14068f9c13fa052c8fa60f63912cd5781409d7cd 100644
--- a/pkg/http/test/client_test.dart
+++ b/pkg/http/test/client_test.dart
@@ -41,7 +41,7 @@ void main() {
       'body': '{"hello": "world"}'
     }))));
 
-    request.sink.add('{"hello": "world"}'.charCodes);
+    request.sink.add('{"hello": "world"}'.codeUnits);
     request.sink.close();
   });
 
@@ -54,7 +54,7 @@ void main() {
 
     expect(client.send(request), throwsSocketIOException);
 
-    request.sink.add('{"hello": "world"}'.charCodes);
+    request.sink.add('{"hello": "world"}'.codeUnits);
     request.sink.close();
   });
 }