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 276a4af1d9791c61fd80a69e7fdeb660a8d4fe75..2dd5420fe66d335b2737d41819864d26e84f6c4d 100644
--- a/utils/pub/validator.dart
+++ b/utils/pub/validator.dart
@@ -18,6 +18,7 @@ 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
@@ -52,7 +53,8 @@ abstract class Validator {
       new PubspecFieldValidator(entrypoint),
       new DependencyValidator(entrypoint),
       new DirectoryValidator(entrypoint),
-      new CompiledDartdocValidator(entrypoint)
+      new CompiledDartdocValidator(entrypoint),
+      new Utf8ReadmeValidator(entrypoint)
     ];
 
     return Future.wait(validators.map((validator) => validator.validate()))