Fix a crash in pub's version solver.

sdk/lib/_internal/pub/lib/src/solver/version_selection.dart

Index: sdk/lib/_internal/pub/lib/src/solver/version_selection.dart
diff --git a/sdk/lib/_internal/pub/lib/src/solver/version_selection.dart b/sdk/lib/_internal/pub/lib/src/solver/version_selection.dart
index cabf450951536cdf748d34f124d1054d4761f855..c815f9f75ba62292dab7615a148f10d6cc8c1ae4 100644
--- a/sdk/lib/_internal/pub/lib/src/solver/version_selection.dart
+++ b/sdk/lib/_internal/pub/lib/src/solver/version_selection.dart
@@ -68,12 +68,6 @@ class VersionSelection {
       // unselected queue.
       if (deps.length == 1 && dep.name != _solver.root.name) {
         await _unselected.add(dep.toRef());
-
-        // If the package depends on barback, add pub's implicit dependency on
-        // barback and related packages as well.
-        if (dep.name == 'barback') {
-          await _unselected.add(new PackageRef.magic('pub itself'));
-        }
       }
     });
   }
@@ -89,12 +83,6 @@ class VersionSelection {
 
       if (deps.isEmpty) {
         _unselected.remove(dep.toRef());
-
-        // If this was the last package that depended on barback, get rid of
-        // pub's implicit dependency.
-        if (dep.name == 'barback') {
-          _unselected.remove(new PackageRef.magic('pub itself'));
-        }
       }
     }
   }