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

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

Index: sdk/lib/_internal/pub/lib/src/command/global_list.dart
diff --git a/sdk/lib/_internal/pub/lib/src/command/global_list.dart b/sdk/lib/_internal/pub/lib/src/command/global_list.dart
new file mode 100644
index 0000000000000000000000000000000000000000..c498b42545ba87e2c1ae911866cda2036145b927
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+++ b/sdk/lib/_internal/pub/lib/src/command/global_list.dart
@@ -0,0 +1,21 @@
+// Copyright (c) 2014, 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.
+
+library pub.command.global_list;
+
+import 'dart:async';
+
+import '../command.dart';
+
+/// Handles the `global list` pub command.
+class GlobalListCommand extends PubCommand {
+  bool get allowTrailingOptions => false;
+  String get description => 'List globally activated packages.';
+  String get usage => 'pub global list';
+
+  Future onRun() {
+    globals.listActivePackages();
+    return null;
+  }
+}