code: int resolveFieldIndex(VariableElement field)
      {
          TypeElement declaringClass = (TypeElement) field.getEnclosingElement();
          String descriptor = Descriptor.getDesriptor(field);
          int index = resolveFieldIndex(declaringClass, field.getSimpleName().toString(), descriptor);
          addIndexedElement(index, field);
          return index;
      }
docs: Returns the constant map index to field.
  If entry doesn't exist it is created.
  @param field
  @return