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

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

Index: sdk/lib/_internal/pub/lib/src/command/cache.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/cache.dart b/sdk/lib/_internal/pub/lib/src/command/cache.dart
index dbef31531b5743f95a357e5e78e780f4870b3ac8..93897fae11ecfb7679a44c667959aaa51e13efa7 100644
--- a/sdk/lib/_internal/pub/lib/src/command/cache.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/cache.dart
@@ -5,6 +5,7 @@
 library pub.command.cache;
 
 import '../command.dart';
+import 'cache_add.dart';
 import 'cache_list.dart';
 
 /// Handles the `cache` pub command.
@@ -13,6 +14,7 @@ class CacheCommand extends PubCommand {
   String get usage => "pub cache <subcommand>";
 
   final subcommands = {
+    "add": new CacheAddCommand(),
     "list": new CacheListCommand()
   };
 }