If a tab is closed before the method responds, the Messenger throws "Could not establish connection"

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Context:

• Incorrect "connection lost" message for multi-page flows pixiebrix-extension#2902 (comment)
• Closing the receiver tab will not throw an error in tabs.sendMessage() mozilla/webextension-polyfill#365

Refer to the steps in pixiebrix/pixiebrix-extension#2902 (comment)

Possible solution:

• Drop polyfill usage

Drawbacks:

• We don't get native Errors in Firefox (🤷‍♂️)

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This problem is temporarily fixed by 0d20b5a but if this issue is resolved:

• A listener indicated an asynchronous response by returning true, but the message channel closed before a response was received mozilla/webextension-polyfill#384

before this one:

• Closing the receiver tab will not throw an error in tabs.sendMessage() mozilla/webextension-polyfill#365

the problem will return.

I think the best way to handle this is to drop the polyfill so we don't have to depend on what Mozilla’s schedule and decisions.

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Reopening because of my previous comment