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 e101f282762beaf932ba5dbf3bd3c7865e60d1b5..658acb51b6dcb1bb01ee9cae5f747bb0d8923c50 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();
     }), completes);
   });
@@ -56,7 +56,7 @@ void main() {
 
       expect(client.send(request), throwsSocketIOException);
 
-      request.sink.add('{"hello": "world"}'.charCodes);
+      request.sink.add('{"hello": "world"}'.codeUnits);
       request.sink.close();
     }), completes);
   });