guava-gwt/src-super/com/google/common/collect/super/com/google/common/collect/ImmutableSortedMap.java
private static <K, V> SortedMap<K, V> newModifiableDelegate(Comparator<? super K> comparator) {
    return newTreeMap(nullAccepting(comparator));
  }
private static <K, V> SortedMap<K, V> newModifiableDelegate(Comparator<? super K> comparator) {
    return newTreeMap(nullAccepting(comparator));
  }
/*
We don't permit nulls, but we wrap every comparator with nullsFirst().
Why? We want for queries like containsKey(null) to return false, but the
GWT SortedMap implementation that we delegate to throws
NullPointerException if the comparator does. Since our construction
methods ensure that null is never present in the map, it's OK for the
comparator to look for it wherever it wants.

Note that we do NOT touch the comparator returned by comparator(), which
should be identical to the one the user passed in. We touch only the
"secret" comparator used by the delegate implementation.