Types like output(T), promise(T), and union(T_0, ..., T_N) should be
assignable from dynamic if they contain an element type that is
assignable from dynamic.
This allows consumers to use the following code to check if some type
behaves like the dynamic type w.r.t. conversions:
```
if t.AssignableFrom(model.DynamicType) {
}
```
Fixes#4703.
05032d6850