Remove synthetic package qualifier for flutter: generate: error. (#163145)

`generateLocalizations` should fail (`flutter gen-l10n`) if `flutter:
generate:` does not exist.

The previous logic was faulty, because it was totally possible to
opt-out of synthetic packages (i.e. in a `l10n.yaml` file), but still
not be specifying `flutter: generate:`, which I _believe_ is supposed to
still be an error.

This came up in https://github.com/flutter/flutter/pull/160289 as
`flutter config --explicit-package-dependencies` is enabled by default,
as the error is no longer thrown. Made a few other small test
forward-fixes that otherwise would break with the switch (but are
expected) as well.

dev/benchmarks/test_apps/stocks/pubspec.yaml

@@ -78,6 +78,7 @@ dev_dependencies:
   yaml: 3.1.3 # THIS LINE IS AUTOGENERATED - TO UPDATE USE "flutter update-packages --force-upgrade"
 
 flutter:
+  generate: true
   uses-material-design: true
 
 # PUBSPEC CHECKSUM: 17a3