Add a Pub validator for READMEs that are invalid utf-8.

utils/pub/validator.dart

Index: utils/pub/validator.dart
diff --git a/utils/pub/validator.dart b/utils/pub/validator.dart
index 5ec0f233f5a808ba6c2a72b4dd68abb0acae1a86..acbb02b06992388818e8a2d1041ee3653e698e1c 100644
--- a/utils/pub/validator.dart
+++ b/utils/pub/validator.dart
@@ -17,6 +17,7 @@ import 'validator/lib.dart';
 import 'validator/license.dart';
 import 'validator/name.dart';
 import 'validator/pubspec_field.dart';
+import 'validator/utf8_readme.dart';
 
 /// The base class for validators that check whether a package is fit for
 /// uploading. Each validator should override [errors], [warnings], or both to
@@ -50,7 +51,8 @@ abstract class Validator {
       new NameValidator(entrypoint),
       new PubspecFieldValidator(entrypoint),
       new DependencyValidator(entrypoint),
-      new DirectoryValidator(entrypoint)
+      new DirectoryValidator(entrypoint),
+      new Utf8ReadmeValidator(entrypoint)
     ];
 
     return Future.wait(validators.mappedBy((validator) => validator.validate()))