Dart Quake App - Issues with Async

import 'package:flutter/material.dart';
import 'dart:async';
import 'dart:convert';
import 'package:http/http.dart' as http;
import 'package:intl/intl.dart';

class Quake extends StatefulWidget {
  @override
  State<StatefulWidget> createState() => new QuakeState();
}

class QuakeState extends State<Quake> {
    // https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_day.geojson
    final String url = "https://earthquake.usgs.gov/earthquakes/feed/v1.0/summary/all_day.geojson";

  @override
  Widget build(BuildContext context) {
    List earthquakes;
    var _data;

    return new Scaffold(
        appBar: new AppBar(title: new Text("Quakes - USGS All Earthquakes"),
        backgroundColor: Colors.red,
        centerTitle: true,),
        body: new ListView.builder(
            itemBuilder: (BuildContext context, int index) {
              _data = getJson(url).then((onValue){
                earthquakes = onValue['features'];

              });
              var time = earthquakes != null ? earthquakes[index]['properties']['time']: "";
              time *= 1000;
              var dateTime = new DateTime.fromMillisecondsSinceEpoch(int.parse(time));
              time = new DateFormat.yMMMMd(dateTime).add_jm();
              return new ListTile(
                title: new Text(time ?? ""),
              );
            }));
  }
}

Future<Map> getJson(String url) async {
  var response = await http.get(url);
  return json.decode(response.body);
}