Make @keepToString annotation work for web and non-web

[mkustermann]

• Update documentation for the annotation
• Add @keepToString annotation to web's Locale class (similar to non-webs Locale class)

Issue flutter/flutter#52759

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[osa1]

It looks like the same pragma is defined multiple times, with duplicated documentation, except one of them has a usage example and the other one is not.

Is there a way to move the pragma to a common dependency and document it once there?

[mkustermann]

It looks like the same pragma is defined multiple times, with duplicated documentation, except one of them has a usage example and the other one is not.

Is there a way to move the pragma to a common dependency and document it once there?

They both have the usage example actually.

The duplication arises from the fact that the dart:ui implementation of the normal flutter engine and web engine are completely separated, they don't seem to share any code atm.

I don't want this PR to start creating a dependency from lib/web_ui/lib on vm version in lib/ui/. It wouldn't be very clean either because the annotations.dart is not a dart library but rather a part of ui, it would be very strange if that part is used as part of two separate libraries. So I decided to keep it like this.