Dynamically populate SPFX web part property dropdown list - using lists in current site. See http://cob-sp.com/SPFx-WP-Props2

FetchListsForWebPartPropsDropdown.ts

/* need some imports e.g.:
import { IODataList } from '@microsoft/sp-odata-types';
import { SPHttpClient, SPHttpClientConfigurations, SPHttpClientConfiguration, SPHttpClientResponse, ODataVersion, ISPHttpClientConfiguration } from '@microsoft/sp-http';
*/

private dropdownOptions: IPropertyPaneDropdownOption[];
private listsFetched: boolean;

// these methods are split out to go step-by-step, but you could refactor and be more direct if you choose..

private fetchLists(url: string) : Promise<any> {
    return this.context.spHttpClient.get(url, SPHttpClient.configurations.v1).then((response: SPHttpClientResponse) => {
      if (response.ok) {
        return response.json();
      } else {
        console.log("WARNING - failed to hit URL " + url + ". Error = " + response.statusText);
        return null;
      }
    });
}

private fetchOptions(): Promise<IPropertyPaneDropdownOption[]> {
  var url = this.context.pageContext.web.absoluteUrl + `/_api/web/lists?$filter=Hidden eq false`;

  return this.fetchLists(url).then((response) => {
      var options: Array<IPropertyPaneDropdownOption> = new Array<IPropertyPaneDropdownOption>();
      response.value.map((list: IODataList) => {
          console.log("Found list with title = " + list.Title);
          options.push( { key: list.Id, text: list.Title });
      });

      return options;
  });
}