Fix analyzer errors/warnings for Pub.

utils/pub/yaml/composer.dart

Index: utils/pub/yaml/composer.dart
diff --git a/utils/pub/yaml/composer.dart b/utils/pub/yaml/composer.dart
index c10e5e818c6ed17bb752237db3fa8e998b096498..e88253dfe5744cab6f6515ee03131a335db0cabe 100644
--- a/utils/pub/yaml/composer.dart
+++ b/utils/pub/yaml/composer.dart
@@ -2,6 +2,8 @@
 // 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.
 
+part of yaml;
+
 /**
  * Takes a parsed YAML document (what the spec calls the "serialization tree")
  * and resolves aliases, resolves tags, and parses scalars to produce the