pub: treat a pubspec with just comments as empty

dart/utils/tests/pub/pubspec_test.dart

Index: dart/utils/tests/pub/pubspec_test.dart
diff --git a/dart/utils/tests/pub/pubspec_test.dart b/dart/utils/tests/pub/pubspec_test.dart
index ddc5d51496432d8f43f7bc4ce3d3cbd5c279437a..ca4364aff7d7707df8ca962e1708d1bbdf2a57a9 100644
--- a/dart/utils/tests/pub/pubspec_test.dart
+++ b/dart/utils/tests/pub/pubspec_test.dart
@@ -53,6 +53,20 @@ dependencies:
 ''', sources);
         });
       });
+
+      test("allows comment-only files", () {
+        var sources = new SourceRegistry();
+        sources.register(new MockSource());
+
+        var pubspec = new Pubspec.parse('''
+# No external dependencies yet
+# Including for completeness
+# ...and hoping the spec expands to include details about author, version, etc
+# See http://www.dartlang.org/docs/pub-package-manager/ for details
+''', sources);
+        expect(pubspec.version, equals(Version.none));
+        expect(pubspec.dependencies, isEmpty);
+      });
     });
   });
 }