In flutter/plugins#3318 we introduced a mechanism to skip tests for the nnbd pluging on stable, determining whether to skip the plugins is done by looking at the CHANNEL environment variable set by Cirrus.

We had critical problems in that PR:

1. The comparison done with [[ "$CHANNEL" -eq "stable" ]] always returned true, which means we essentially stopped testing the nnbd plugins. (Not a bash expert but I believe -eq is for integer comparisons, this should have been a == comparison).
2. Some CI tasks are only running on stable which means for these tasks we won't be testing the nnbd plugins.

Unfortunately this resulted in bad code successfully passing CI and landing for example: the new tests in flutter/plugins#3334 introduced Dart compile errors, this was not detected by the analyze phase as the plugin was completely skipped in https://cirrus-ci.com/task/4598922096148480, from the log of that task:

./script/incremental_build.sh analyze
Excluding the following plugins: android_intent,connectivity,device_info,flutter_plugin_android_lifecycle,flutter_webview,google_sign_in,local_auth,path_provider,plugin_platform_interface,share,url_launcher,video_player,webview_flutter

cc @blasten @bparrishMines