Support "pub downgrade".

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

Index: sdk/lib/_internal/pub/test/test_pub.dart
diff --git a/sdk/lib/_internal/pub/test/test_pub.dart b/sdk/lib/_internal/pub/test/test_pub.dart
index 0650075cd3ec9ec029f4c0e7630cc8a75bcd8ef1..1bbf30716558b80ec23a70b3ae1d5ec1a4d758f2 100644
--- a/sdk/lib/_internal/pub/test/test_pub.dart
+++ b/sdk/lib/_internal/pub/test/test_pub.dart
@@ -329,6 +329,8 @@ class RunCommand {
       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;
@@ -388,6 +390,12 @@ void pubUpgrade({Iterable<String> args, output, error, warning, int exitCode}) {
       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.
 ///