import 'package:http/http.dart' as http;

 void main() {
  var url = 'http://httpbin.org/';
  http.get(url).then((response) {
    print("Response status: ${response.statusCode}");
    print("Response body: ${response.body}");
  });
}

錯誤提示：

Your application could not be compiled, because its dependencies could not be established.

The following Dart file:

/Users/mac/APP/http_test/lib/main.dart

...refers, in an import, to the following library:

package:http/http.dart

That library is in a package that is not known. Maybe you forgot to mention it in your pubspec.yaml file?

原因：

需要在專案根目錄下的檔案pubspec.yaml中註冊依賴：

dependencies:
  http: ^0.11.0      // --加入這行 
  flutter:
    sdk: flutter