Add "pub global list" command to show all globally activated packages.

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

Index: sdk/lib/_internal/pub/lib/src/command/global.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/global.dart b/sdk/lib/_internal/pub/lib/src/command/global.dart
index 8c2105ca6ef019568f37584970d7f3687d94d8a1..c1b17302f6fd0a2e627a4c943f2b8b01651d2d78 100644
--- a/sdk/lib/_internal/pub/lib/src/command/global.dart
+++ b/sdk/lib/_internal/pub/lib/src/command/global.dart
@@ -7,6 +7,7 @@ library pub.command.global;
 import '../command.dart';
 import 'global_activate.dart';
 import 'global_deactivate.dart';
+import 'global_list.dart';
 import 'global_run.dart';
 
 /// Handles the `global` pub command.
@@ -17,6 +18,7 @@ class GlobalCommand extends PubCommand {
   final subcommands = {
     "activate": new GlobalActivateCommand(),
     "deactivate": new GlobalDeactivateCommand(),
+    "list": new GlobalListCommand(),
     "run": new GlobalRunCommand()
   };
 }