Changed interfaces to abstract classes in utils.

utils/pub/version_solver.dart

 // Copyright (c) 2012, the Dart project authors.  Please see the AUTHORS file
 // for details. All rights reserved. Use of this source code is governed by a
 // BSD-style license that can be found in the LICENSE file.
 
 /**
  * Attempts to resolve a set of version constraints for a package dependency
  * graph and select an appropriate set of best specific versions for all
  * dependent packages. It works iteratively and tries to reach a stable
  * solution where the constraints of all dependencies are met. If it fails to
  * reach a solution after a certain number of iterations, it assumes the
@@ skipping 202 matching lines @@
     });
   }
 }
 
 /**
  * The constraint solver works by iteratively processing a queue of work items.
  * Each item is a single atomic change to the dependency graph. Handling them
  * in a queue lets us handle asynchrony (resolving versions requires information
  * from servers) as well as avoid deeply nested recursion.
 */
-interface WorkItem {
+abstract class WorkItem {
   /**
    * Processes this work item. Returns a future that completes when the work is
    * done. If `null` is returned, that means the work has completed
    * synchronously and the next item can be started immediately.
    */
   Future process(VersionSolver solver);
 }
 
 /**
  * The best selected version for a package has changed to [version]. If the
@@ skipping 466 matching lines @@
   final description1;
   final description2;
 
   DescriptionMismatchException(this.package, this.description1,
       this.description2);
 
   // TODO(nweiz): Dump to YAML when that's supported
   String toString() => "Package '$package' has conflicting descriptions "
     "'${JSON.stringify(description1)}' and '${JSON.stringify(description2)}'";
 }