Change the host_debug sky_shell used for testing to build with assertions enabled

scripts/slave/recipes/flutter/engine.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]"
     ],
     "name": "rmcontents everything",
     "~followup_annotations": [
@@ skipping 120 matching lines @@
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
     },
     "name": "download android tools"
   },
   {
     "cmd": [
       "[SLAVE_BUILD]/src/flutter/tools/gn",
       "--runtime-mode",
-      "debug"
+      "debug",
+      "--unoptimized"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
     },
-    "name": "gn --runtime-mode debug"
+    "name": "gn --runtime-mode debug --unoptimized"
   },
   {
     "cmd": [
       "[SLAVE_BUILD]/src/flutter/tools/gn",
       "--runtime-mode",
       "profile",
       "--android"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
     },
     "name": "gn --runtime-mode profile --android"
   },
   {
     "cmd": [
       "[SLAVE_BUILD]/src/flutter/tools/gn",
       "--runtime-mode",
       "release",
       "--android"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
     },
     "name": "gn --runtime-mode release --android"
   },
   {
     "cmd": [
       "ninja",
       "-C",
-      "[SLAVE_BUILD]/src/out/host_debug"
+      "[SLAVE_BUILD]/src/out/host_debug_unopt"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
     },
-    "name": "build host_debug"
+    "name": "build host_debug_unopt"
   },
   {
     "cmd": [
       "ninja",
       "-C",
       "[SLAVE_BUILD]/src/out/android_profile",
       "flutter/lib/snapshot"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
@@ skipping 15 matching lines @@
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::zip]/resources/zip.py"
     ],
     "env": {
       "PATH": "[SLAVE_BUILD]/src/third_party/dart-sdk/dart-sdk/bin:%(PATH)s"
     },
     "name": "Zip darwin-x64 artifacts.zip",
-    "stdin": "{\"entries\": [{\"archive_name\": \"sky_snapshot\", \"path\": \"[SLAVE_BUILD]/src/out/host_debug/sky_snapshot\", \"type\": \"file\"}, {\"archive_name\": \"sky_shell\", \"path\": \"[SLAVE_BUILD]/src/out/host_debug/sky_shell\", \"type\": \"file\"}, {\"archive_name\": \"icudtl.dat\", \"path\": \"[SLAVE_BUILD]/src/out/host_debug/icudtl.dat\", \"type\": \"file\"}], \"output\": \"[TMP_BASE]/tmp_tmp_1/artifacts.zip\", \"root\": \"[SLAVE_BUILD]/src\"}"
+    "stdin": "{\"entries\": [{\"archive_name\": \"sky_snapshot\", \"path\": \"[SLAVE_BUILD]/src/out/host_debug_unopt/sky_snapshot\", \"type\": \"file\"}, {\"archive_name\": \"sky_shell\", \"path\": \"[SLAVE_BUILD]/src/out/host_debug_unopt/sky_shell\", \"type\": \"file\"}, {\"archive_name\": \"icudtl.dat\", \"path\": \"[SLAVE_BUILD]/src/out/host_debug_unopt/icudtl.dat\", \"type\": \"file\"}], \"output\": \"[TMP_BASE]/tmp_tmp_1/artifacts.zip\", \"root\": \"[SLAVE_BUILD]/src\"}"
   },
   {
     "cmd": [
       "python",
       "-u",
       "RECIPE_MODULE[build::gsutil]/resources/gsutil_wrapper.py",
       "--",
       "RECIPE_PACKAGE_REPO[depot_tools]/gsutil.py",
       "----",
       "cp",
@@ skipping 476 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
   }
 ]