Remove deprecated string features.

samples/chat/chat_server_lib.dart

Index: samples/chat/chat_server_lib.dart
diff --git a/samples/chat/chat_server_lib.dart b/samples/chat/chat_server_lib.dart
index b9845322d691f5f9b3f95f97b50778243950f830..c78d60496fad7b0c1a359c6f2d8210af5bf6749d 100644
--- a/samples/chat/chat_server_lib.dart
+++ b/samples/chat/chat_server_lib.dart
@@ -317,7 +317,7 @@ class IsolatedServer {
                            HttpResponse response,
                            String redirectPath) {
     if (_redirectPage == null) {
-      _redirectPage = redirectPageHtml.charCodes;
+      _redirectPage = redirectPageHtml.codeUnits;
     }
     response.statusCode = HttpStatus.FOUND;
     response.headers.set(
@@ -361,7 +361,7 @@ class IsolatedServer {
   // Serve the not found page.
   void _notFoundHandler(HttpRequest request, HttpResponse response) {
     if (_notFoundPage == null) {
-      _notFoundPage = notFoundPageHtml.charCodes;
+      _notFoundPage = notFoundPageHtml.codeUnits;
     }
     response.statusCode = HttpStatus.NOT_FOUND;
     response.headers.set("Content-Type", "text/html; charset=UTF-8");