Issue with comparison operator in generated service worker

[KernCheh]

Description

There is an bug I believe with the comparison operator in the generated service worker for Flutter web. When the service worker registration receives a message event, it always triggers the downloadOffline case due to the assignment operator being used in place of the comparison operator.

I really appreciate the detailed guidance on contributing guidelines, however, I apologize in advance that I might not have the bandwidth to look into much follow-ups on this. The motivation of putting in this PR is to highlight the issue :)

Please feel free to merge if this is a bug, or create a separate contribution if there are any legal issues, many thanks!

[fluttergithubbot]

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[KernCheh]

This should not be = I believe

[KernCheh]

@googlebot I signed it!

[googlebot]

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[jonahwilliams]

LGTM

Thanks for the fix