Increase Dart's github polling interval, especially for client.dart.packages.

BUG=

Review URL: https://codereview.chromium.org/1395563005

git-svn-id: svn://svn.chromium.org/chrome/trunk/tools/build@297078 0039d316-1c4b-4281-b951-d872f2087c98
3 files changed