Another example: this code

void main() {
  dynamic f;
  List x;
  var a = x.map(f);
  int b = a;
}

Produces an error indicating that a is being inferred as Iterable<Object>. I suspect that the greatest closure computation is being done filling in with Object instead of dynamic. For this example, the downwards context on f is dynamic -> ? so the choice of what to fill in for ? in the greatest closure computation would result in Iterable<Object> or Iterable<dynamic> for Object/dynamic respectively.

This issue breaks dart2js on Google's tests. A workaround has been committed but it will affect other users so I'm giving it a high priority to figure out what's going on, how easy it is to change, and what other effects that will have.

A bit of context for the fix. Absence of handling of defaultType in some Kernel visitors resulted in some mistakes in distinguishing between omitted bounds and Object provided explicitly for the bounds. In its turn, it resulted in more type variables being marked as having explicit Object as their bound. It was exactly the case for the examples above: Object was provided because it was assumed the explicit bound of the type variable provided by the programmer.

Thanks for the fix! Came across another test failure due to this.