setup the correct version of Xcode

scripts/slave/recipes/flutter/flutter.expected/mac.json

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\nfrom common import chromium_utils # Error? See https://crbug.com/584783.\n\npath = sys.argv[1]\nif os.path.exists(path):\n  for p in (os.path.join(path, x) for x in os.listdir(path)):\n    if os.path.isdir(p):\n      chromium_utils.RemoveDirectory(p)\n    else:\n      os.unlink(p)\n",
       "[SLAVE_BUILD]"
     ],
     "cwd": "[SLAVE_BUILD]",
     "name": "rmcontents everything",
@@ skipping 87 matching lines @@
       "submodule",
       "update",
       "--init",
       "--recursive"
     ],
     "cwd": "[SLAVE_BUILD]/flutter",
     "name": "submodule update"
   },
   {
     "cmd": [
-      "[SLAVE_BUILD]/flutter/infra/download_android_tools.py"
+      "python",
+      "-u",
+      "[BUILD]/scripts/slave/ios/find_xcode.py",
+      "--json-file",
+      "/path/to/tmp/json"
     ],
     "cwd": "[SLAVE_BUILD]",
-    "name": "download android tools"
+    "env": {
+      "ANDROID_HOME": "[SLAVE_BUILD]/flutter/infra/android_tools",
+      "PATH": "[SLAVE_BUILD]/flutter/bin:[SLAVE_BUILD]/flutter/bin/cache/dart-sdk/bin:%(PATH)s",
+      "PUB_CACHE": "[SLAVE_BUILD]/.pub-cache"
+    },
+    "name": "enumerate_xcode_installations",
+    "~followup_annotations": [
+      "@@@STEP_LOG_LINE@json.output@{@@@",
+      "@@@STEP_LOG_LINE@json.output@  \"installations\": {@@@",
+      "@@@STEP_LOG_LINE@json.output@    \"/some/path\": \"7.2.1 build_number\"@@@",
+      "@@@STEP_LOG_LINE@json.output@  }@@@",
+      "@@@STEP_LOG_LINE@json.output@}@@@",
+      "@@@STEP_LOG_END@json.output@@@"
+    ]
   },
   {
     "cmd": [
+      "python",
+      "-u",
+      "[BUILD]/scripts/slave/ios/find_xcode.py",
+      "--json-file",
+      "/path/to/tmp/json",
+      "--version",
+      "7.2.1"
+    ],
+    "cwd": "[SLAVE_BUILD]",
+    "env": {
+      "ANDROID_HOME": "[SLAVE_BUILD]/flutter/infra/android_tools",
+      "PATH": "[SLAVE_BUILD]/flutter/bin:[SLAVE_BUILD]/flutter/bin/cache/dart-sdk/bin:%(PATH)s",
+      "PUB_CACHE": "[SLAVE_BUILD]/.pub-cache"
+    },
+    "name": "set_xcode_version",
+    "~followup_annotations": [
+      "@@@STEP_LOG_LINE@json.output@{}@@@",
+      "@@@STEP_LOG_END@json.output@@@"
+    ]
+  },
+  {
+    "cmd": [
       "flutter",
       "doctor"
     ],
     "cwd": "[SLAVE_BUILD]",
     "env": {
       "ANDROID_HOME": "[SLAVE_BUILD]/flutter/infra/android_tools",
       "PATH": "[SLAVE_BUILD]/flutter/bin:[SLAVE_BUILD]/flutter/bin/cache/dart-sdk/bin:%(PATH)s",
       "PUB_CACHE": "[SLAVE_BUILD]/.pub-cache"
     },
     "name": "flutter doctor"
@@ skipping 166 matching lines @@
       "@@@STEP_LOG_LINE@python.inline@  chromium_utils.RemoveDirectory(sys.argv[1])@@@",
       "@@@STEP_LOG_END@python.inline@@@"
     ]
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]