See the component page for more information.
Import files are a new invention, so libraries like d3 do not yet provide them.
d3-import is an intermediary that provides an import file for the d3 component.
d3-import depends on d3.
Components that want to use d3 should depend on d3-element and import d3-import to be safe from library duplication.
Such components need not use Polymer or d3-element, but we put the import and the element in one package for convenience.