Switch to mirror for third-party/markdown checkout.

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",
 
   # The svn location to pull out dependencies from
@@ skipping 204 matching lines @@
       Var("http_throttle_rev"),
   Var("dart_root") + "/third_party/pkg/intl":
       (Var("github_mirror") % "intl") + Var("intl_rev"),
   Var("dart_root") + "/third_party/pkg/json_rpc_2":
       (Var("github_mirror") % "json_rpc_2") + Var("json_rpc_2_tag"),
   Var("dart_root") + "/third_party/pkg/linter":
       (Var("github_mirror") % "linter") + Var("linter_tag"),
   Var("dart_root") + "/third_party/pkg/logging":
       (Var("github_mirror") % "logging") + Var("logging_rev"),
   Var("dart_root") + "/third_party/pkg/markdown":
-      "https://github.com/dart-lang/" +
-      "markdown.git" + Var("markdown_rev"),
+      (Var("github_mirror") % "markdown") + Var("markdown_rev"),
   Var("dart_root") + "/third_party/pkg/matcher":
       (Var("github_mirror") % "matcher") + Var("matcher_tag"),
   Var("dart_root") + "/third_party/pkg/metatest":
       (Var("github_mirror") % "metatest") + Var("metatest_rev"),
   Var("dart_root") + "/third_party/pkg/mime":
       (Var("github_mirror") % "mime") + Var("mime_rev"),
   Var("dart_root") + "/third_party/pkg/mustache4dart":
       Var("chromium_git")
       + "/external/github.com/valotas/mustache4dart.git"
       + Var("mustache4dart_rev"),
@@ skipping 135 matching lines @@
       '--no_resume',
       '--bucket',
       'dart-dependencies',
       '-d',
       '-r',
       Var('dart_root') + '/third_party/d8',
     ],
   },
 
 ]