Elide `platform_channels`, `platform_design` & `testing_app` from Beta CI (#1129)

tool/flutter_ci_script_beta.sh

@@ -32,12 +32,15 @@ declare -ar PROJECT_NAMES=(
     "null_safety/null_safe_app"
     "null_safety/null_unsafe_app"
     "place_tracker"
-    "platform_channels"
+    # TODO(domesticmouse): Re-enable once Dart 2.17 is stable
-    "platform_design"
+    # "platform_channels"
+    # TODO(domesticmouse): Re-enable once Dart 2.17 is stable
+    # "platform_design"
     "platform_view_swift"
     "provider_counter"
     "provider_shopper"
-    "testing_app"
+    # TODO(domesticmouse): Re-enable once Dart 2.17 is stable
+    # "testing_app"
     "veggieseasons"
 )