The class 'Future' was not exported from 'dart:core' until version 2.1, but this code is required...

dart-warning.png

当前pubspec.yaml

  environment:
  sdk: ">=2.0.0-dev.68.0 <3.0.0"

修改为:

environment:
  sdk: ">=2.1.0 <3.0.0"

然后package get,即可解决

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