Switch pkg packages, pub, and dartdoc to use package: imports.

sdk/lib/_internal/dartdoc/lib/universe_serializer.dart

Index: sdk/lib/_internal/dartdoc/lib/universe_serializer.dart
diff --git a/sdk/lib/_internal/dartdoc/lib/universe_serializer.dart b/sdk/lib/_internal/dartdoc/lib/universe_serializer.dart
index b88c9ead7749a9031930839b8a194d3514561c5e..edeca41b0e744df5dda5a61b57164d404d02fcdb 100755
--- a/sdk/lib/_internal/dartdoc/lib/universe_serializer.dart
+++ b/sdk/lib/_internal/dartdoc/lib/universe_serializer.dart
@@ -11,7 +11,7 @@ library universe_serializer;
 import 'dartdoc.dart';
 
 // TODO(rnystrom): Use "package:" URL (#4968).
-import '../../../../../pkg/pathos/lib/path.dart' as path;
+import 'package:pathos/path.dart' as path;
 import '../../compiler/implementation/mirrors/dart2js_mirror.dart' as dart2js;
 import '../../compiler/implementation/mirrors/mirrors.dart';
 import '../../compiler/implementation/mirrors/mirrors_util.dart';