src/app/dialogs/alert/alert.component.ts

import { Component, Optional } from '@angular/core';
import { MdDialogRef } from '@angular/material';

@Component({
  selector: 'app-alert',
  templateUrl: './alert.component.html',
  styleUrls: ['./alert.component.css']
})
export class AlertComponent {
  alertMessage = '';

  constructor(@Optional() public dialogRef: MdDialogRef<AlertComponent>) { }

}