pub: treat a pubspec with just comments as empty

BUG= http://dartbug.com/3483

TEST=

Review URL: https://chromiumcodereview.appspot.com//10534081
Patch from Kevin Moore <kevmoo@j832.com>.

git-svn-id: https://dart.googlecode.com/svn/branches/bleeding_edge@9686 260f80e4-7a28-3924-810f-c04153c831b5

lib/src/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.');
     }

test/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);
+      });
     });
   });
 }