Type parameter bounds can be marked "exact nullable" if upcast.

[MichaelRFairhurst]

class B<T extends Object> {                                                      
  void f(T t) {                                                                  
    // Create an edge from the bound to some type                                
    List<dynamic> x = [t];                                                       
    // and make that type exact nullable                                         
    x[0] = null;                                                                 
  }                                                                              
}

This migration should produce no source edits.

Currently, the assignment from List<T> to List<dynamic> results in an edge from the bound of T to the dynamic type argument in List<dynamic>. This is correct.

But it has an unfortunate consequence, it means that when dynamic is marked exact nullable (by assigning null to the list index), that exact nullability is propagated upstream, and that is currently not stopped in any way from marking the bound of T as exact nullable.

We therefore get T extends Object? whis is unnecessary.

This occurs in package:async.

[MichaelRFairhurst]

Related to #40552