title: How to export const with NgModule
body: <p>I have one constant which should be usable by other modules importing mine.</p>
  
  <p>I have Datepicker directive which has defined date formats constants (I have it like this so someone using my directive can use these constants):</p>
  
  <pre><code>export const DATETIME_FORMAT = 'DD/MM/YYYY HH:mm:ss';
  export const DATE_FORMAT = 'DD/MM/YYYY';
  export const TIME_FORMAT = 'DD/MM/YYYY';
  </code></pre>
  
  <p>And NgModule for exporting them further. This module is built and then used in other projects/modules.</p>
  
  <pre><code>import { NgModule } from '@angular/core';
  import { CommonModule } from '@angular/common';
  import { DatepickerDirective, TIME_FORMAT, DATETIME_FORMAT, DATE_FORMAT } from './datepicker.directive';
  
  @NgModule({
    imports: [
      CommonModule
    ],
    exports: [
      DatepickerDirective,
      DATE_FORMAT,
      DATETIME_FORMAT,
      TIME_FORMAT
    ],
    declarations: [
      DatepickerDirective
    ]
  })
  export class DatepickerModule { }
  </code></pre>
  
  <p>But when trying to build it I get error:</p>
  
  <pre><code>ERROR in src/app/datepicker/datepicker.module.ts(5,11): error TS2345: Argument of type '{ imports: (typeof CommonModule)[]; exports: (string | typeof DatepickerDirective)[]; declara...' is not assignable to parameter of type 'NgModule'.
    Types of property 'exports' are incompatible.
      Type '(string | typeof DatepickerDirective)[]' is not assignable to type '(any[] | Type&lt;any&gt;)[]'.
        Type 'string | typeof DatepickerDirective' is not assignable to type 'any[] | Type&lt;any&gt;'.
          Type 'string' is not assignable to type 'any[] | Type&lt;any&gt;'.
  </code></pre>