anchor.diff:      // Move onto the next package if we've traversed all of these references.
       if (refs.isEmpty) return _traversePackage();
   
  +    // Pump the event loop to flatten the stack trace and workaround #9583.
  +    // If that bug is fixed, this can be Future.sync() instead.
       return new Future(() {
         var ref = refs.removeFirst();
   
  
positive.code:           // If it fails for any reason, just treat that as no versions. This
            // will sort this reference higher so that we can traverse into it
            // and report the error more properly.
            return new Pair<PackageRef, int>(ref, 0);
          });
        }
  
negative.code:   /// Returns a string describing the dependency chain if it does, or `null` if
    /// there is no dependency.
    String _getDependencyPath(PackageId depender, String dependent) {
      // TODO(rnystrom): This is O(n^2) where n is the number of selected
      // packages. Could store the reverse dependency graph to address that. If
      // we do that, we need to make sure it gets correctly rolled back when
      // backtracking occurs.
      var visited = new Set<String>();
  
      walkDeps(PackageId package, String currentPath) {
        if (visited.contains(package.name)) return null;
        visited.add(package.name);
  
        var pubspec = cache.getCachedPubspec(package);
        if (pubspec == null) return null;
  
        for (var dep in pubspec.dependencies) {
          if (dep.name == dependent) return currentPath;
  
          var selected = getSelected(dep.name);
          // Ignore unselected dependencies. We haven't traversed into them yet,
          // so they can't affect backjumping.
          if (selected == null) continue;
  
          var depPath = walkDeps(selected, '$currentPath -> ${dep.name}');
          if (depPath != null) return depPath;
        }
  
        return null;
      }
  
      return walkDeps(depender, depender.name);
    }
  
    /// Logs [message] in the context of the current selected packages. If
  