如果你需要模拟一个异常，这两种方法都可以工作：
1.模拟调用并为expect函数提供一个预期在执行后会抛出的函数（如果任何测试在expect函数之外抛出异常，Mockito似乎会自动失败）：

when(mockInstance.foo(input))
  .thenThrow(ArgumentError);

expect(
  () => mockInstance.foo(input), // or just mockInstance.foo(input)
  throwsArgumentError,
);

1.如果这是一个异步调用，并且你在try-catch块上捕获异常并返回一些东西，你可以使用then.Answer：

when(mockInstance.foo(input))
  .thenAnswer((_) => throw ArgumentError());

final a = await mockInstance.foo(input);

// assert
verify(...);
expect(...)

1.如果异常不是由mock抛出的：

expect(
  methodThatThrows(),
  throwsA(isA<YourCustomException>()),
);

我也遇到了同样的问题。试试看

expect(() => mockInstance.foo(input), throwsArgumentError);

下面是一个所有测试都通过的示例类

import 'package:flutter_test/flutter_test.dart';
import 'package:mockito/mockito.dart';

void main() {
  test("",(){
    var mock = new MockA();
    when(mock.foo1()).thenThrow(new ArgumentError());

    expect(() => mock.foo1(), throwsArgumentError);
  });

  test("",(){
    var mock = new MockA();
    when(mock.foo2()).thenThrow(new ArgumentError());

    expect(() => mock.foo2(), throwsArgumentError);
  });

  test("",(){
    var mock = new MockA();
    when(mock.foo3()).thenThrow(new ArgumentError());

    expect(() => mock.foo3(), throwsArgumentError);
  });

  test("",(){
    var mock = new MockA();
    when(mock.foo4()).thenThrow(new ArgumentError());

    expect(() => mock.foo4(), throwsArgumentError);
  });
}

class MockA extends Mock implements A {}

class A {
  void foo1() {}
  int foo2() => 3;
  Future foo3() async {}
  Future<int> foo4() async => Future.value(3);
}