Add stack chain support to pub.

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

Index: sdk/lib/_internal/pub/lib/src/validator/compiled_dartdoc.dart
diff --git a/sdk/lib/_internal/pub/lib/src/validator/compiled_dartdoc.dart b/sdk/lib/_internal/pub/lib/src/validator/compiled_dartdoc.dart
index d6abf6d7f707fddc5c87ca70dc44001c547e397e..400e6b2271b402c3c6b8bc02a4cb0f3efd402ed9 100644
--- a/sdk/lib/_internal/pub/lib/src/validator/compiled_dartdoc.dart
+++ b/sdk/lib/_internal/pub/lib/src/validator/compiled_dartdoc.dart
@@ -10,6 +10,7 @@ import 'package:path/path.dart' as path;
 
 import '../entrypoint.dart';
 import '../io.dart';
+import '../utils.dart';
 import '../validator.dart';
 
 /// Validates that a package doesn't contain compiled Dartdoc
@@ -19,7 +20,7 @@ class CompiledDartdocValidator extends Validator {
     : super(entrypoint);
 
   Future validate() {
-    return new Future.sync(() {
+    return syncFuture(() {
       for (var entry in listDir(entrypoint.root.dir, recursive: true)) {
         if (path.basename(entry) != "nav.json") continue;
         var dir = path.dirname(entry);