Fix some compilation errors and warnings in Pub.

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 429 matching lines @@
 
   RemoveConstraint(this.depender, this.dependent);
 
   Future process(VersionSolver solver) {
     var dependency = solver.getDependency(dependent);
     var oldDependency = dependency.clone();
     _removed = dependency.removeConstraint(depender);
     return _processChange(solver, oldDependency, dependency);
   }
 
-  void undo() {
+  void undo(VersionSolver solver) {
     solver.getDependency(dependent).placeConstraint(depender, _removed);
   }
 }
 
 /** [package]'s version is no longer constrained by the lockfile. */
 class UnlockPackage implements WorkItem {
   /** The package being unlocked. */
   Dependency package;
 
   UnlockPackage(this.package);
@@ skipping 239 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)}'";
 }