Support "pub downgrade".

sdk/lib/_internal/pub/test/test_pub.dart

 // Copyright (c) 2013, 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.
 
 /// Test infrastructure for testing pub.
 ///
 /// Unlike typical unit tests, most pub tests are integration tests that stage
 /// some stuff on the file system, run pub, and then validate the results. This
 /// library provides an API to build tests like that.
 library test_pub;
@@ skipping 311 matching lines @@
 /// Set to true when the current batch of scheduled events should be aborted.
 bool _abortScheduled = false;
 
 /// Enum identifying a pub command that can be run with a well-defined success
 /// output.
 class RunCommand {
   static final get = new RunCommand('get', new RegExp(
       r'Got dependencies!|Changed \d+ dependenc(y|ies)!'));
   static final upgrade = new RunCommand('upgrade', new RegExp(
       r'(No dependencies changed\.|Changed \d+ dependenc(y|ies)!)$'));
+  static final downgrade = new RunCommand('downgrade', new RegExp(
+      r'(No dependencies changed\.|Changed \d+ dependenc(y|ies)!)$'));
 
   final String name;
   final RegExp success;
   RunCommand(this.name, this.success);
 }
 
 /// Runs the tests defined within [callback] using both pub get and pub upgrade.
 ///
 /// Many tests validate behavior that is the same between pub get and
 /// upgrade have the same behavior. Instead of duplicating those tests, this
@@ skipping 39 matching lines @@
 void pubGet({Iterable<String> args, output, error, warning, int exitCode}) {
   pubCommand(RunCommand.get, args: args, output: output, error: error,
       warning: warning, exitCode: exitCode);
 }
 
 void pubUpgrade({Iterable<String> args, output, error, warning, int exitCode}) {
   pubCommand(RunCommand.upgrade, args: args, output: output, error: error,
       warning: warning, exitCode: exitCode);
 }
 
+void pubDowngrade({Iterable<String> args, output, error, warning,
+    int exitCode}) {
+  pubCommand(RunCommand.downgrade, args: args, output: output, error: error,
+      warning: warning, exitCode: exitCode);
+}
+
 /// Schedules starting the "pub [global] run" process and validates the
 /// expected startup output.
 ///
 /// If [global] is `true`, this invokes "pub global run", otherwise it does
 /// "pub run".
 ///
 /// Returns the `pub run` process.
 ScheduledProcess pubRun({bool global: false, Iterable<String> args}) {
   var pubArgs = global ? ["global", "run"] : ["run"];
   pubArgs.addAll(args);
@@ skipping 603 matching lines @@
         _lastMatcher.matches(item.last, matchState);
   }
 
   Description describe(Description description) {
     return description.addAll("(", ", ", ")", [_firstMatcher, _lastMatcher]);
   }
 }
 
 /// A [StreamMatcher] that matches multiple lines of output.
 StreamMatcher emitsLines(String output) => inOrder(output.split("\n"));