import(name, {referrerName, address, metadata} = {}) {
    return this.internalLoader_.load(name, referrerName, address, metadata).
        then((codeUnit) => this.get(codeUnit.normalizedName));
  }
import - Asynchronously load, link, and evaluate a module and any
dependencies it imports.
@param {string} name, ModuleSpecifier-like name, not normalized.
@return {Promise.<Module>}