Flow type for AssertContext.throws should match typescript type

[dancoates]

Description

The flow type for AssetContext.throws specifies that the return value is Promise<Error>:

type AssertContext = {
    ...
    throws: {
        (value: PromiseLike<mixed>, error?: ErrorValidator, message?: string): Promise<Error>;
        (value: () => mixed, error?: ErrorValidator, message?: string): Error;
    };
    ...
};

This is inconsistent with the typescript types:

export interface AssertContext {
    ...
    throws(value: PromiseLike<any>, error?: ErrorValidator, message?: string): Promise<any>;
    throws(value: () => void, error?: ErrorValidator, message?: string): any;
    ...
}

and causes issues with flow when your code throws something other than an Error object.

Test Source

//@flow
import test from 'ava';

class CustomError extends Error {
    customProperty: string;
    constructor(message: string, customValue: string) {
        super(message);
        this.customProperty = customValue;
    }
}

function throwsCustomError(): Promise<string> {
    return new Promise((resolve, reject) => {
        setTimeout(() => {
            reject(new CustomError('message', 'customValue'));
        }, 500);
    });
}

test('Test something', async (tt: AssertContext): Promise<void> => {
    const error = await tt.throws(throwsCustomError());
    tt.is(error.customProperty, 'customValue');
});

Error Message & Stack Trace

test.js:22
 22:     tt.is(error.customProperty, 'customValue');
                     ^^^^^^^^^^^^^^ property `customProperty`. Property not found in
 22:     tt.is(error.customProperty, 'customValue');
               ^^^^^ Error


Found 1 error
error Command failed with exit code 2.

Config

Copy the relevant section from package.json:

{
  "ava": {
    "babel": "inherit"
  }
}

Relevant Links

Minimal repro

I'll submit a PR in a min with a fix

[novemberborn]

@dancoates do you think we should make throws() generic so you can type the return value? Or is any with type casting sufficient?