pub: treat a pubspec with just comments as empty

utils/pub/pubspec.dart

Index: utils/pub/pubspec.dart
diff --git a/utils/pub/pubspec.dart b/utils/pub/pubspec.dart
index 04c942e60887de8c34c4708704345e2b70cbf279..94b03f239f956998a28b7b0d99a84f54e7166efc 100644
--- a/utils/pub/pubspec.dart
+++ b/utils/pub/pubspec.dart
@@ -42,6 +42,10 @@ class Pubspec {
     if (contents.trim() == '') return new Pubspec.empty();
 
     var parsedPubspec = loadYaml(contents);
+    if (parsedPubspec == null) {

[nweiz, 2012/07/16 19:34:02]

Nit: we usually do single-line if statements when they're just returning a value
(as above).

+      // content just yaml comments
+      return new Pubspec.empty();
+    }
     if (parsedPubspec is! Map) {
       throw new FormatException('The pubspec must be a YAML mapping.');
     }