Add a Pub source for pub.dartlang.org.

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index 855206830b47f7611f821ecd11c3e23ff08b050b..7b3e66644c6ef368f7eeb24d19aceda6e1a42d7f 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -141,6 +141,6 @@ class Entrypoint {
     // once that's implemented. If the lockfile doesn't exist, it should
     // generate it. In the meantime, here's a dumb implementation:
     return new Future.immediate(
-        new PackageId(ref.source, Version.none, ref.description));
+        new PackageId(ref.source, ref.constraint, ref.description));
   }
 }