Switch Dart's pub-mac slave to a mac 10.8 machine.
Switch Dart's pub-mac slave to a mac 10.8 machine.

BUG=

Review URL: https://codereview.chromium.org/1537563005

git-svn-id: 239fca9b83025a0b6f823aeeca02ba5be3d9fd76@298055 0039d316-1c4b-4281-b951-d872f2087c98

eunchong/build,eunchong/build,eunchong/build,eunchong/build