Update DEPS to use github mirror for two more repos

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 128 matching lines @@
     Var("clang_format_rev"),
 
   Var("dart_root") + "/tests/co19/src":
       (Var("github_mirror") % "co19") + Var("co19_rev"),
 
   Var("dart_root") + "/third_party/zlib":
       Var("chromium_git") + "/chromium/src/third_party/zlib.git" +
       Var("zlib_rev"),
 
   Var("dart_root") + "/third_party/boringssl":
-     (Var("github_dartlang") % "boringssl_gen") + Var("boringssl_gen_rev"),
+      (Var("github_mirror") % "boringssl_gen") + Var("boringssl_gen_rev"),
   Var("dart_root") + "/third_party/boringssl/src":
       "https://boringssl.googlesource.com/boringssl.git" +
       Var("boringssl_rev"),
 
   Var("dart_root") + "/third_party/root_certificates":
       (Var("github_mirror") % "root_certificates") +
       Var("root_certificates_rev"),
 
   Var("dart_root") + "/third_party/jinja2":
       Var("chromium_git") + "/chromium/src/third_party/jinja2.git" +
@@ skipping 87 matching lines @@
       (Var("github_mirror") % "metatest") + Var("metatest_tag"),
   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_tag"),
   Var("dart_root") + "/third_party/pkg/oauth2":
       (Var("github_mirror") % "oauth2") + Var("oauth2_tag"),
   Var("dart_root") + "/third_party/pkg/observable":
-      (Var("github_dartlang") % "observable") + Var("observable_tag"),
+      (Var("github_mirror") % "observable") + Var("observable_tag"),
   Var("dart_root") + "/third_party/pkg/observe":
       (Var("github_mirror") % "observe") + Var("observe_tag"),
   Var("dart_root") + "/third_party/observatory_pub_packages":
-     (Var("github_mirror") % "observatory_pub_packages")
+      (Var("github_mirror") % "observatory_pub_packages")
       + Var("observatory_pub_packages_rev"),
   Var("dart_root") + "/third_party/pkg_tested/package_config":
       (Var("github_mirror") % "package_config") +
       Var("package_config_tag"),
   Var("dart_root") + "/third_party/pkg_tested/package_resolver":
       (Var("github_mirror") % "package_resolver") + Var("package_resolver_tag"),
   Var("dart_root") + "/third_party/pkg/path":
       (Var("github_mirror") % "path") + Var("path_tag"),
   Var("dart_root") + "/third_party/pkg/plugin":
       (Var("github_mirror") % "plugin") + Var("plugin_tag"),
@@ skipping 322 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"],
   },
 ]