Improving string conversion

[FMorschel]

Fixes #40888

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[FMorschel]

Requested @bwilkerson to review on Gerrit, as he wrote the assist.

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[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/384962 has been updated with the latest commits from this pull request.

[copybara-service]

https://dart-review.googlesource.com/c/sdk/+/384962 has been updated with the latest commits from this pull request.

[copybara-service]

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