Downstream impact from https://codereview.chromium.org/1693993002/

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

 [
   {
     "cmd": [
       "python",
       "-u",
       "\nimport os, sys\nfrom common import chromium_utils\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 43 matching lines @@
       "checkout",
       "-f",
       "FETCH_HEAD"
     ],
     "cwd": "[SLAVE_BUILD]/flutter",
     "name": "git checkout"
   },
   {
     "cmd": [
       "git",
+      "rev-parse",
+      "HEAD"
+    ],
+    "cwd": "[SLAVE_BUILD]/flutter",
+    "name": "read revision",
+    "stdout": "/path/to/tmp/",
+    "~followup_annotations": [
+      "@@@STEP_TEXT@<br/>checked out 'deadbeef'<br/>@@@"
+    ]
+  },
+  {
+    "cmd": [
+      "git",
       "clean",
       "-f",
       "-d",
       "-x"
     ],
     "cwd": "[SLAVE_BUILD]/flutter",
     "name": "git clean"
   },
   {
     "cmd": [
@@ skipping 118 matching lines @@
       "PUB_CACHE": "[SLAVE_BUILD]/.pub-cache"
     },
     "name": "test packages/newton"
   },
   {
     "name": "$result",
     "recipe_result": null,
     "status_code": 0
   }
 ]