Make VersionSolver source- and description-aware.

utils/pub/entrypoint.dart

Index: utils/pub/entrypoint.dart
diff --git a/utils/pub/entrypoint.dart b/utils/pub/entrypoint.dart
index 7b3e66644c6ef368f7eeb24d19aceda6e1a42d7f..deb4afa168fe455185a9a36df9ae0f1a5ace13f5 100644
--- a/utils/pub/entrypoint.dart
+++ b/utils/pub/entrypoint.dart
@@ -140,7 +140,6 @@ class Entrypoint {
     // TODO(rnystrom): This should use the lockfile to select the right version
     // 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, ref.constraint, ref.description));
+    return new Future.immediate(ref.atVersion(ref.constraint));
   }
 }