Update DEPS to linter `0.1.21`.

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 52 matching lines @@
   "html_tag" : "@0.12.1+1",
   "http_multi_server_tag" : "@2.0.0",
   "http_parser_tag" : "@3.0.2",
   "http_tag" : "@0.11.3+3",
   "http_throttle_rev" : "@a81f08be942cdd608883c7b67795c12226abc235",
   "idl_parser_rev": "@7fbe68cab90c38147dee4f48c30ad0d496c17915",
   "initialize_rev": "@595d501a92c3716395ad2d81f9aabdb9f90879b6",
   "intl_tag": "@0.13.0",
   "jinja2_rev": "@2222b31554f03e62600cd7e383376a7c187967a1",
   "json_rpc_2_tag": "@2.0.0",
-  "linter_rev": "@1a08f395b1f4e24b3901018938320f053bad6aa8",
+  "linter_rev": "@7ca3aab6ca45b988440e425c187993a533fbe27e",
   "logging_rev": "@85d83e002670545e9039ad3985f0018ab640e597",
   "markdown_rev": "@4aaadf3d940bb172e1f6285af4d2b1710d309982",
   "matcher_tag": "@0.12.0",
   "metatest_rev": "@e5aa8e4e19fc4188ac2f6d38368a47d8f07c3df1",
   "mime_rev": "@75890811d4af5af080351ba8a2853ad4c8df98dd",
   "mustache4dart_rev" : "@5724cfd85151e5b6b53ddcd3380daf188fe47f92",
   "oauth2_tag": "@1.0.0",
   "observatory_pub_packages_rev": "@cf90eb9077177d3d6b3fd5e8289477c2385c026a",
   "observe_rev": "@eee2b8ec34236fa46982575fbccff84f61202ac6",
   "package_config_rev": "@0.1.5",
@@ skipping 390 matching lines @@
       "--extract",
       "-s",
       Var('dart_root') + "/third_party/clang.tar.gz.sha1",
     ],
   },
   {
     "pattern": ".",
     "action": ["python", Var("dart_root") + "/tools/gyp_dart.py"],
   },
 ]