Add a "cache add" command to download packages to the cache.

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

Index: sdk/lib/_internal/pub/lib/src/command.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command.dart b/sdk/lib/_internal/pub/lib/src/command.dart
index 3f6f35e13f63143fea65a945e9d83edd6bed7dcc..d5eb6eabef02e3226ffb7e730a550f997fb03b39 100644
--- a/sdk/lib/_internal/pub/lib/src/command.dart
+++ b/sdk/lib/_internal/pub/lib/src/command.dart
@@ -181,6 +181,8 @@ abstract class PubCommand {
   }
 
   /// Displays usage information for this command.
+  ///
+  /// If [description] is omitted, defaults to the command's description.
   void printUsage([String description]) {
     if (description == null) description = this.description;
     log.message('$description\n\n${_getUsage()}');