sdk_version_async_exported_from_core

The class '{0}' wasn't exported from 'dart:core' until version 2.1, but this code is required to be able to run on earlier versions.

The analyzer produces this diagnostic when either the class Future or Stream is referenced in a library that doesn't import dart:async in code that has an SDK constraint whose lower bound is less than 2.1.0. In earlier versions, these classes weren't defined in dart:core, so the import was necessary.

Here's an example of a pubspec that defines an SDK constraint with a lower bound of less than 2.1.0:

environment:
  sdk: '>=2.0.0 <2.4.0'

In the package that has that pubspec, code like the following produces this diagnostic:

void f(Future f) {}

If you don't need to support older versions of the SDK, then you can increase the SDK constraint to allow the classes to be referenced:

environment:
  sdk: '>=2.1.0 <2.4.0'

If you need to support older versions of the SDK, then import the dart:async library.

import 'dart:async';

void f(Future f) {}

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