Support "pub downgrade".

sdk/lib/_internal/pub/lib/src/global_packages.dart

 // Copyright (c) 2014, 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.
 
 library pub.global_packages;
 
 import 'dart:async';
-import 'dart:convert';
 import 'dart:io';
 
 import 'package:path/path.dart' as p;
 
 import 'entrypoint.dart';
 import 'io.dart';
 import 'lock_file.dart';
 import 'log.dart' as log;
 import 'package.dart';
 import 'system_cache.dart';
@@ skipping 52 matching lines @@
 
     var package;
     var id;
     return _selectVersion(name, currentVersion, constraint).then((version) {
       // Make sure it's in the cache.
       id = new PackageId(name, _source.name, version, name);
       return _source.downloadToSystemCache(id);
     }).then((p) {
       package = p;
       // Resolve it and download its dependencies.
-      return resolveVersions(cache.sources, package, lockFile: lockFile);
+      return resolveVersions(SolveType.GET, cache.sources, package,
+          lockFile: lockFile);
     }).then((result) {
       if (!result.succeeded) throw result.error;
-      result.showReport();
+      result.showReport(SolveType.GET);
 
       // Make sure all of the dependencies are locally installed.
       return Future.wait(result.packages.map((id) {
         var source = cache.sources[id.source];
         if (source is! CachedSource) return new Future.value();
         return source.downloadToSystemCache(id)
             .then((_) => source.resolveId(id));
       }));
     }).then((ids) {
       var lockFile = new LockFile(ids);
@@ skipping 79 matching lines @@
 
       // Pick the best matching version.
       versions.sort(Version.prioritize);
       return versions.last;
     });
   }
 
   /// Gets the path to the lock file for an activated package with [name].
   String _getLockFilePath(name) => p.join(_directory, name + ".lock");
 }