A couple options

reflection.js

    Polymer.__importLazyGroup = function(
        callingElement, group, fromElement, options) {
      if (!fromElement) {
        fromElement = Polymer.DomModule.import(callingElement.localName);
      }

      var importStatuses = {loaded: [], failed: []};
      var groupAttribute = group ? '[group=' + group + ']' : ':not([group])';
      var query =
        'link' +groupAttribute+ '[rel=\'lazy-import\'][href]:not([href=\'\'])';
      var links = fromElement.querySelectorAll(query);

      if (links.length < 1) {
        return Promise.resolve(importStatuses);
      }

      var promises = Array.from(links).map(function(link) {
        var href = link.getAttribute('href');
        var resolvedUrl = callingElement.resolveUrl(href);

        var reflect = function(state) {
          return state;
        };
        var getPromise = function() {
          if (__importPromises.hasOwnProperty(resolvedUrl)) {
            if (options && !options.retry) {
              return __importPromises[resolvedUrl].catch(reflect);
            } else {
              return __importPromises[resolvedUrl];
            }
          } else {
            return Promise.reject();
          }
        };

        return getPromise().catch(function() {
          return __importPromises[resolvedUrl] =
              new Promise(function(resolve, reject) {
            var onLoad = function() {
              return resolve({loaded: href});
            };
            var onFail = function() {
              return reject({failed: href});
            };
            var importHref = Polymer.importHref || callingElement.importHref;
            importHref(resolvedUrl, onLoad, onFail);
          });
        }).catch(reflect);
      });

      return Promise.all(promises).then(function(importRequests) {
        return importRequests.reduce(function(requests, nextRequest) {
          if (nextRequest.loaded) {
            requests.loaded.push(nextRequest.loaded);
          }
          if (nextRequest.failed) {
            requests.failed.push(nextRequest.failed);
          }
          return requests;
        }, importStatuses);
      });
    };

standard.js

    Polymer.__importLazyGroup = function(
        callingElement, group, fromElement, options) {
      if (!fromElement) {
        fromElement = Polymer.DomModule.import(callingElement.localName);
      }

      var importStatuses = {loaded: [], failed: []};
      var groupAttribute = group ? '[group=' + group + ']' : ':not([group])';
      var query =
        'link' +groupAttribute+ '[rel=\'lazy-import\'][href]:not([href=\'\'])';
      var links = fromElement.querySelectorAll(query);

      if (links.length < 1) {
        return Promise.resolve(importStatuses);
      }

      var promises = Array.from(links).map(function(link) {
        var promise;
        var href = link.getAttribute('href');
        var resolvedUrl = callingElement.resolveUrl(href);

        var requestImport = function() {
          return new Promise(function(resolve, reject) {
            var importHref = Polymer.importHref || callingElement.importHref;
            importHref(resolvedUrl, resolve, reject);
          });
        };

        if (__importPromises.hasOwnProperty(resolvedUrl)) {
          promise = __importPromises[resolvedUrl];
          if (options && options.retry) {
            promise = promise.catch(requestImport);
          }
        } else {
          promise = requestImport();
        }

        __importPromises[resolvedUrl] = promise;

        return promise.then(
          function onLoad() {
            return {loaded: href};
          },
          function onFail() {
            return {failed: href};
          }
        );
      });

      return Promise.all(promises).then(function(importRequests) {
        return importRequests.reduce(function(requests, nextRequest) {
          if (nextRequest.loaded) {
            requests.loaded.push(nextRequest.loaded);
          }
          if (nextRequest.failed) {
            requests.failed.push(nextRequest.failed);
          }
          return requests;
        }, importStatuses);
      });
    };