[Wildcard Variables] Web debugging support

[kallentu]

This issue tracks the web debugging support needed for the wildcards feature.

References

Correct - I think the nuance here is that in the code we emit in JavaScript we need to introduce a placeholder name to generate a valid function signature.

While our intent was to eventually make dwds model all of its state based on what we know from the dart program, there were cases in the past where it created its internal model based on the Chrome debugger's state of the application. If that is still the case today, we may need to do some work to ensure those synthetic variables are properly ignored/hidden.

Originally posted by @sigmundch in #55751

cc. @nshahan @bkonyi