pub: treat a pubspec with just comments as empty

dart/utils/pub/pubspec.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // 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.
 
 #library('pubspec');
 
 #import('package.dart');
 #import('source.dart');
 #import('source_registry.dart');
 #import('utils.dart');
@@ skipping 24 matching lines @@
    * Parses the pubspec whose text is [contents]. If the pubspec doesn't define
    * version for itself, it defaults to [Version.none].
    */
   factory Pubspec.parse(String contents, SourceRegistry sources) {
     var version = Version.none;
     var dependencies = <PackageRef>[];
 
     if (contents.trim() == '') return new Pubspec.empty();
 
     var parsedPubspec = loadYaml(contents);
+    if (parsedPubspec == null) return new Pubspec.empty();
+
     if (parsedPubspec is! Map) {
       throw new FormatException('The pubspec must be a YAML mapping.');
     }
 
     if (parsedPubspec.containsKey('version')) {
       version = new Version.parse(parsedPubspec['version']);
     }
 
     if (parsedPubspec.containsKey('dependencies')) {
       var dependencyEntries = parsedPubspec['dependencies'];
@@ skipping 41 matching lines @@
         source.validateDescription(description);
 
         dependencies.add(new PackageRef(
             source, versionConstraint, description));
       });
     }
 
     return new Pubspec(version, dependencies);
   }
 }