Note: 'http.dart' is a composable, Future-based library for making HTTP requests.

main.dart

Future<List> getCurrencies() async {
  String apiUrl = 'https://api.coinmarketcap.com/v1/ticker/?limit=50';
  // Make a HTTP GET request to the CoinMarketCap API.
  // Await basically pauses execution until the get() function returns a Response
  http.Response response = await http.get(apiUrl);
  // Using the JSON class to decode the JSON String
  return JSON.decode(response.body);
}

JSON is deprecated. I'm now using json instead.