Step 1: git pull the newest code of flutter engine (commid id : 56cddc7)
Step 2: do gclient sync and other op ...
Step 3: /flutter/tools/gn --android --runtime-mode=profile
show WARNING as fellow:

The figure shows, impeller_enable_playground is invalid when gn gen android.
We can see the code in flutter/BUILD.gn

  if (is_mac) {
      public_deps += [
        "//flutter/impeller:impeller_unittests",
        "//flutter/shell/platform/darwin:flutter_channels_unittests",
      ]
    }

Only if the compiled engine product is macos, it will depend on //flutter/impeller:impeller_unittests , which introduces the impeller_enable_playground.