update pub to latest

DEPS

 # Copyright (c) 2012, 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.
 
 vars = {
   # The dart_root is the root of our sdk checkout. This is normally
   # simply sdk, but if using special gclient specs it can be different.
   "dart_root": "sdk",
 
   # We use mirrors of all github repos to guarantee reproducibility and
@@ skipping 77 matching lines @@
   "observable_tag": "@0.17.0",
   "observatory_pub_packages_rev": "@26aad88f1c1915d39bbcbff3cad589e2402fdcf1",
   "observe_tag": "@0.15.0",
   "package_config_tag": "@1.0.0",
   "package_resolver_tag": "@1.0.2+1",
   "path_tag": "@1.4.1",
   "plugin_tag": "@0.2.0",
   "ply_rev": "@604b32590ffad5cbb82e4afef1d305512d06ae93",
   "pool_tag": "@1.3.0",
   "protobuf_tag": "@0.5.4",
-  "pub_rev": "@492372c4546c5a1a8b45891f956d2b2e98221fb0",
+  "pub_rev": "@a8781274e3a38c34a49d4c5bffb98557c6022a75",
   "pub_semver_tag": "@1.3.2",
   "quiver_tag": "@0.22.0",
   "resource_rev":"@a49101ba2deb29c728acba6fb86000a8f730f4b1",
   "root_certificates_rev": "@a4c7c6f23a664a37bc1b6f15a819e3f2a292791a",
   "scheduled_test_tag": "@0.12.11",
   "shelf_static_tag": "@0.2.4",
   "shelf_packages_handler_tag": "@1.0.0",
   "shelf_tag": "@0.6.7+2",
   "shelf_web_socket_tag": "@0.2.1",
   "smoke_tag" : "@v0.3.6+2",
@@ skipping 383 matching lines @@
     # Update the Windows toolchain if necessary.
     'name': 'win_toolchain',
     'pattern': '.',
     'action': ['python', 'sdk/build/vs_toolchain.py', 'update'],
   },
   {
     "pattern": ".",
     "action": ["python", Var("dart_root") + "/tools/generate_buildfiles.py"],
   },
 ]