pub: treat a pubspec with just comments as empty

dart/utils/pub/pubspec.dart

Index: dart/utils/pub/pubspec.dart
diff --git a/dart/utils/pub/pubspec.dart b/dart/utils/pub/pubspec.dart
index d3a5bb8358969f383f7c7386b68a910b90f392b4..767a79176cfdd2a0812be2ba4ff8c7a33fac9354 100644
--- a/dart/utils/pub/pubspec.dart
+++ b/dart/utils/pub/pubspec.dart
@@ -42,6 +42,8 @@ class Pubspec {
     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.');
     }