MegaMenu doesn't compile with TypeScript 2.4

[Cito]

I'm submitting a ...

[X] bug report

Test case

You can use the following demo app as test case:
https://github.com/ova2/angular-development-with-primeng/tree/master/chapter7/megamenu

Current behavior
If you run the showcase for the MegaMenu with TypeScript 2.4 or run the demo app linked above, you will get a compilation error like

 Type '{ label: string; items: { label: string; }[]; }[]' has no properties in common with type 'MenuItem'.

Expected behavior
There should be no compilation error, as when compiling with TypeScript 2.3.

Minimal reproduction of the problem with instructions

• Install the above app

• or install the current master of PrimeNG and change the requirements in package.json to Angular 4.3, Angular-Cli 1.3, TypeScript 2.4 (lower Angular/Cli versions require TypeScript 2.3, so you need to test with Angular 4.3 and Cli 1.3)

• Run npm install and npm start and check the MegaMenu

• Angular version: 4.3.3

• PrimeNG version: 4.1.2

• Browser: all

• Language: TypeScript 2.4

• Node (for AoT issues): 8.1.4

• Analysis of the problem:

In the MenuItem interface, the items property is defined as of type MenuItem[]. But in the MegaMenu, you can have arrays of arrays of MenuItems as items, not just arrays of MenuItems.

In TypeScript 2.4, it’s now an error to assign anything to a weak type when there’s no overlap in properties (see here).

Note that in the MenuItem interface all properties are marked as optional. Therefore, this is considered a "weak type".

• Proposed solution:

The items property in the MenutItem interface should be defined as follows:

items?: MenuItem[]|MenuItem[][];

[Guoye]

发现 version: 4.2.2 也存在相同的问题，以下是showcase的编译错误:

Minimal reproduction of the problem with instructions

Angular version: 4.4.4

PrimeNG version: 4.2.2

Analysis of the problem:

Failed to compile.

primeng/src/app/components/common/messageservice.ts (21,37): Type 'Message[]' has no properties in common with type 'Message'.

primeng/src/app/showcase/components/megamenu/megamenudemo.ts (12,9):Type '{ label: string; icon: string; items: { label: string; items: { label: string; }[]; }[][]; }[]' is not assignable to type 'MenuItem[]' ....

[corvinrok]

Why is this not included in the latest 5.0.0-rc release?