Exception: TypeError: data.map(...).flat is not a function

[eddex]

Environment
coverage-viewer: 0.4.1
OS: Windows 10
shell: PowerShell
npm: 6.13.0
node: v10.16.3

Install output

> npm i -g coverage-viewer

npm WARN deprecated array.prototype.flatten@1.2.1: This method has been renamed to "flat" - please use `array.prototype.flat` instead
C:\Users\larry\AppData\Roaming\npm\coverage-viewer -> C:\Users\larry\AppData\Roaming\npm\node_modules\coverage-viewer\cli.js
+ coverage-viewer@0.4.1
added 108 packages from 65 contributors in 10.354s

Exception

> coverage-viewer "./tests/coverage.json" -s "./src" -o "./viewer"

C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\statistics.js:1
const getAllFiles = data => data.map(project => project.files).flat()
                                                               ^

TypeError: data.map(...).flat is not a function
    at getAllFiles (C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\statistics.js:1:64)
    at exports.getFileStats.data (C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\statistics.js:93:17)
    at renderFile (C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\render.js:34:21)
    at files.map.file (C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\render.js:52:13)
    at Array.map (<anonymous>)
    at Object.exports.renderFiles (C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\render.js:49:16)
    at fs.readFile (C:\Users\me\AppData\Roaming\npm\node_modules\coverage-viewer\lib\render.js:73:13)
    at FSReqWrap.readFileAfterClose [as oncomplete] (internal/fs/read_file_context.js:53:3)

[eddex]

Seems to be similar to #10

[derrickwilliams]

Before this commit the array.prototype.flatten module was used to shim the flatten method on array.prototype. But now the native array.prototype.flat method is being used. However, array.prototype.flat is only available in Node 11+.

Along with updating the call to .flat(), the array.prototype.flat shim should be used for backwards compatibility.