Some more removals of dart:utf.

sdk/lib/_internal/pub/lib/src/validator/utf8_readme.dart

 // Copyright (c) 2013, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 library pub.validator.utf8_readme;
 
 import 'dart:async';
-import 'dart:utf';
+import 'dart:convert';
 
 import '../entrypoint.dart';
 import '../io.dart';
 import '../validator.dart';
 
 /// Validates that a package's README is valid utf-8.
 class Utf8ReadmeValidator extends Validator {
   Utf8ReadmeValidator(Entrypoint entrypoint)
     : super(entrypoint);
 
   Future validate() {
     return new Future.sync(() {
       var readme = entrypoint.root.readmePath;
       if (readme == null) return;
       var bytes = readBinaryFile(readme);
       try {
-        // The second and third arguments here are the default values. The
-        // fourth tells [decodeUtf8] to throw an ArgumentError if `bytes` isn't
-        // valid utf-8.
-        decodeUtf8(bytes, 0, null, null);
-      } on ArgumentError catch (_) {
+        // UTF8.decode doesn't allow invalid UTF-8.
+        UTF8.decode(bytes);
+      } on FormatException catch (_) {
         warnings.add("$readme contains invalid UTF-8.\n"
             "This will cause it to be displayed incorrectly on "
                 "pub.dartlang.org.");
       }
     });
   }
 }