V8 Buildbot: Fix dart mac builders.

scripts/slave/recipes/v8.expected/full_client_dart_fyi_v8_mac_release.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/bot_update.py",
       "--master",
       "client.dart.fyi",
       "--builder",
       "v8-mac-release",
@@ skipping 39 matching lines @@
     ]
   },
   {
     "cmd": [
       "python",
       "-u",
       "[DEPOT_TOOLS]/gclient.py",
       "runhooks"
     ],
     "env": {
-      "GYP_DEFINES": "target_arch=ia32 v8_enable_disassembler=1 v8_target_arch=ia32"
+      "GYP_DEFINES": "clang=1 gomadir='[BUILD]/goma' target_arch=ia32 use_goma=1 v8_enable_disassembler=1 v8_target_arch=ia32",
+      "GYP_GENERATORS": "ninja"
     },
     "name": "gclient runhooks"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/cleanup_temp.py"
     ],
     "name": "cleanup_temp"
   },
   {
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/compile.py",
       "--target",
       "Release",
       "--src-dir",
       "[SLAVE_BUILD]/v8",
       "--build-tool",
-      "xcode",
+      "ninja",
+      "--compiler",
+      "goma-clang",
+      "--goma-dir",
+      "[BUILD]/goma",
       "--"
     ],
     "name": "compile"
   },
   {
     "allow_subannotations": true,
     "cmd": [
       "python",
       "-u",
       "[BUILD]/scripts/slave/zip_build.py",
       "--target",
       "Release",
       "--build-url",
       "gs://chromium-v8/v8-for-dart-rel",
       "--src-dir",
       "v8",
       "--factory-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"branch\": \"4.2\", \"buildername\": \"v8-mac-release\", \"buildnumber\": 571, \"mastername\": \"client.dart.fyi\", \"parent_buildername\": null, \"recipe\": \"v8\", \"revision\": \"20123\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}",
       "--build-properties",
       "{\"blamelist\": [\"cool_dev1337@chromium.org\", \"hax@chromium.org\"], \"branch\": \"4.2\", \"buildername\": \"v8-mac-release\", \"buildnumber\": 571, \"mastername\": \"client.dart.fyi\", \"parent_buildername\": null, \"recipe\": \"v8\", \"revision\": \"20123\", \"slavename\": \"TestSlavename\", \"workdir\": \"/path/to/workdir/TestSlavename\"}"
     ],
     "name": "package build"
   }
 ]