Update dart_style to 0.2.12.

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 39 matching lines @@
   "charcode_tag": "@1.1.0",
   "chrome_rev" : "@19997",
   "cli_util_tag" : "@0.0.1+3",
   "code_transformers_tag": "@v0.4.2+3",
   "collection_tag": "@1.10.1",
   "convert_tag": "@2.0.1",
   "crypto_tag" : "@2.0.1",
   "csslib_tag" : "@0.13.2",
   "dart2js_info_tag" : "@0.5.0",
   "dart_services_rev" : "@7aea2574e6f3924bf409a80afb8ad52aa2be4f97",
-  "dart_style_rev": "@a2b84e46a9e4044e61e3cc130087c18da40b9665",
+  "dart_style_tag": "@0.2.13",
   "dartdoc_tag" : "@v0.9.8",
   "fixnum_tag": "@0.10.5",
   "func_tag": "@0.1.0",
   "glob_tag": "@1.1.3",
   "html_tag" : "@0.13.1",
   "http_multi_server_tag" : "@2.0.2",
   "http_parser_tag" : "@3.0.3",
   "http_tag" : "@0.11.3+9",
   "http_throttle_rev" : "@284344cd357e435c6c8ff9a4a21f02b9e384a541",
   "idl_parser_rev": "@7fbe68cab90c38147dee4f48c30ad0d496c17915",
@@ skipping 125 matching lines @@
       (Var("github_mirror") % "crypto") + Var("crypto_tag"),
   Var("dart_root") + "/third_party/pkg/csslib":
       (Var("github_mirror") % "csslib") + Var("csslib_tag"),
   Var("dart_root") + "/third_party/pkg/code_transformers":
       (Var("github_dartlang") % "code_transformers") +
       Var("code_transformers_tag"),
   Var("dart_root") + "/third_party/dart-services":
       (Var("github_mirror") % "dart-services") +
       Var("dart_services_rev"),
   Var("dart_root") + "/third_party/pkg_tested/dart_style":
-      (Var("github_mirror") % "dart_style") + Var("dart_style_rev"),
+      (Var("github_mirror") % "dart_style") + Var("dart_style_tag"),
   Var("dart_root") + "/third_party/pkg/dart2js_info":
       (Var("github_mirror") % "dart2js_info") + Var("dart2js_info_tag"),
   Var("dart_root") + "/third_party/pkg/dartdoc":
       (Var("github_mirror") % "dartdoc") + Var("dartdoc_tag"),
   Var("dart_root") + "/third_party/pkg/func":
       (Var("github_dartlang") % "func") + Var("func_tag"),
   Var("dart_root") + "/third_party/pkg/fixnum":
       "https://github.com/dart-lang/fixnum.git" + Var("fixnum_tag"),
   Var("dart_root") + "/third_party/pkg/glob":
       (Var("github_mirror") % "glob") + Var("glob_tag"),
@@ skipping 375 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"],
   },
 ]