Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/validator/utf8_readme.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/utf8_readme.dart b/sdk/lib/_internal/pub/lib/src/validator/utf8_readme.dart
index 9f30d65599d7668fecd5c13806edeb3d5c30742e..da0c2233ccad333367e95206d22df90df7b0c808 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/utf8_readme.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/utf8_readme.dart
@@ -9,6 +9,7 @@ import 'dart:convert';
 
 import '../entrypoint.dart';
 import '../io.dart';
+import '../utils.dart';
 import '../validator.dart';
 
 /// Validates that a package's README is valid utf-8.
@@ -17,7 +18,7 @@ class Utf8ReadmeValidator extends Validator {
     : super(entrypoint);
 
   Future validate() {
-    return new Future.sync(() {
+    return syncFuture(() {
       var readme = entrypoint.root.readmePath;
       if (readme == null) return;
       var bytes = readBinaryFile(readme);