code: public Nullness getComputedNullness(ExpressionTree e) {
      if (computedNullnessMap.containsKey(e)) {
        return computedNullnessMap.get(e);
      } else {
        return Nullness.NULLABLE;
      }
    }
docs: Returns the computed nullness information from an expression. If none is available, it returns
  Nullable.
  
  <p>Computed information can be added by handlers or by the core, and should supersede that
  comming from annotations.
  
  <p>The default value of an expression without additional computed nullness information is
  always Nullable, since this method should only be called when the fact that the expression is
  NonNull is not clear from looking at annotations.
  
  @param e an expression
  @return computed nullness for e, if any, else Nullable