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

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

Index: scripts/slave/recipes/flutter/engine.expected/mac.json
diff --git a/scripts/slave/recipes/flutter/engine.expected/mac.json b/scripts/slave/recipes/flutter/engine.expected/mac.json
index ef0bded1b86305dd5e181733f4d364be713d17b1..0cdc71f79f0b3d2d64777183631d4ee1a720b9a4 100644
--- a/scripts/slave/recipes/flutter/engine.expected/mac.json
+++ b/scripts/slave/recipes/flutter/engine.expected/mac.json
@@ -138,12 +138,13 @@
     "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": [
@@ -173,12 +174,12 @@
     "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": [
@@ -214,7 +215,7 @@
       "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": [