Download to a temp dir and rename on success.

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 41 matching lines @@
  * Attempts to select the best concrete versions for all of the transitive
  * dependencies of [root] taking into account all of the [VersionConstraint]s
  * that those dependencies place on each other and the requirements imposed by
  * [lockFile]. If successful, completes to a [Map] that maps package names to
  * the selected version for that package. If it fails, the future will complete
  * with a [NoVersionException], [DisjointConstraintException], or
  * [CouldNotSolveException].
  */
 Future<List<PackageId>> resolveVersions(SourceRegistry sources, Package root,
     LockFile lockFile) {
+  print('Resolving dependencies...');
   return new VersionSolver(sources, root, lockFile).solve();
 }
 
 class VersionSolver {
   final SourceRegistry _sources;
   final Package _root;
   final LockFile lockFile;
   final PubspecCache _pubspecs;
   final Map<String, Dependency> _packages;
   final Queue<WorkItem> _work;
@@ skipping 651 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)}'";
 }