Support "pub downgrade".

sdk/lib/_internal/pub/lib/src/command/get.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.
 
 library pub.command.get;
 
 import 'dart:async';
 
 import '../command.dart';
+import '../solver/version_solver.dart';
 
 /// Handles the `get` pub command.
 class GetCommand extends PubCommand {
   String get description => "Get the current package's dependencies.";
   String get usage => "pub get";
   String get docUrl => "http://dartlang.org/tools/pub/cmd/pub-get.html";
   List<String> get aliases => const ["install"];
   bool get isOffline => commandOptions["offline"];
 
   GetCommand() {
     commandParser.addFlag('offline',
         help: 'Use cached packages instead of accessing the network.');
 
     commandParser.addFlag('dry-run', abbr: 'n', negatable: false,
         help: "Report what dependencies would change but don't change any.");
   }
 
   Future onRun() {
-    return entrypoint.acquireDependencies(dryRun: commandOptions['dry-run']);
+    return entrypoint.acquireDependencies(SolveType.GET,
+        dryRun: commandOptions['dry-run']);
   }
 }