The implementation of `com.sun.tools.javac.util.List.map()` looks like this:
```
    public <Z> List<Z> map(Function<A, Z> mapper) {
        boolean changed = false;
        ListBuffer<Z> buf = new ListBuffer<>();
        for (A a : this) {
            Z z = mapper.apply(a);
            buf.append(z);
            changed |= (z != a);
        }
        return changed ? buf.toList() : (List<Z>)this;
    }
```
User Christoph Dreis noticed when profiling compilation tasks that `List.map()` is responsible for very many allocations, however, in many cases the map is empty and any associated allocation is unnecessary.

Therefore, an easy and obvious optimization here would be for `List.map()` to first check if the list is empty, and if so, avoid unnecessarily allocating a `ListBuffer`.