Fix error reporting on invalid command line args.

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 312 matching lines @@
 
 /**
  * A constraint that a depending package places on a dependent package has
  * changed.
  *
  * This is an abstract class that contains logic for updating the dependency
  * graph once a dependency has changed. Changing the dependency is the
  * responsibility of subclasses.
  */
 abstract class ChangeConstraint implements WorkItem {
-  abstract Future process(VersionSolver solver);
+  Future process(VersionSolver solver);
 
-  abstract void undo(VersionSolver solver);
+  void undo(VersionSolver solver);
 
   Future _processChange(VersionSolver solver, Dependency oldDependency,
       Dependency newDependency) {
     var name = newDependency.name;
     var source = oldDependency.source != null ?
       oldDependency.source : newDependency.source;
     var description = oldDependency.description != null ?
       oldDependency.description : newDependency.description;
     var oldConstraint = oldDependency.constraint;
     var newConstraint = newDependency.constraint;
@@ skipping 378 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)}'";
 }