dart2js: Incorrect type variable bounds in argument defaulting stubs

[rakudrama]

Currently we always default to top.

[johnniwinther]

With https://dart-review.googlesource.com/c/sdk/+/59540 we now use the correct bound, except for the cases where the bound uses type variables. In these cases we use "any" type (created for generic js-interop classes) which makes these bounds check pass.

The correct handling of type variables requires a refactoring of how dart2js generate parameter stubs.