Analyzer: No error if library exports declaration 'main' which is not a function

[eernstg]

This is the analyzer specific issue for #43549, which has the details.

[scheglov]

Do we perform these new checks only when null safety is enabled, or always?

[eernstg]

This is a null safety only update (here).

[scheglov]

https://dart-review.googlesource.com/c/sdk/+/165960