Use a lockfile to persist Pub's installed version constellation.

utils/pub/package.dart

Index: utils/pub/package.dart
diff --git a/utils/pub/package.dart b/utils/pub/package.dart
index 46345ed92891f1efd5b2a84e1c3d8e3bb1b52eef..dc5a472dfa7b1c34423efd8f79b4ed8c0e983c1d 100644
--- a/utils/pub/package.dart
+++ b/utils/pub/package.dart
@@ -150,6 +150,11 @@ class PackageId implements Comparable, Hashable {
    * Returns the pubspec for this package.
    */
   Future<Pubspec> describe() => source.describe(this);
+
+  /**
+   * Returns a future that completes to the resovled [PackageId] for this id.
+   */
+  Future<PackageId> get resolved() => source.resolveId(this);
 }
 
 /**