Start pulling pub from its own repo.

sdk/lib/_internal/pub/bin/pub.dart

Index: sdk/lib/_internal/pub/bin/pub.dart
diff --git a/sdk/lib/_internal/pub/bin/pub.dart b/sdk/lib/_internal/pub/bin/pub.dart
deleted file mode 100644
index e781f94785eff1de78ab1e9bbb07ad6e98fc5cb0..0000000000000000000000000000000000000000
--- a/sdk/lib/_internal/pub/bin/pub.dart
+++ /dev/null
@@ -1,9 +0,0 @@
-// Copyright (c) 2013, 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.
-
-import '../lib/src/command_runner.dart';
-
-void main(List<String> arguments) {
-  new PubCommandRunner().run(arguments);
-}