dart2js: `void` arrow setters should allow any rhs type

[eernstg]

This is the dart2js specific issue for #29679. The feature is already supported, so this turned out to be an extension of the test suite only.

[eernstg]

Given that this just adds some tests, we could live with a later milestone. But for now I'll try to get it into 1.24.

[dgrove]

1.24 is very soon. This doesn't look like a hard requirement, so moving to 1.25.

[eernstg]

Landed in 8aef60a, I was just waiting for the tree to be reopened.