Workaround for ngc issue

Author: jelbourn

src/lib/tabs/tab-label.ts

@@ -9,11 +9,14 @@
 import {Directive, TemplateRef, ViewContainerRef} from '@angular/core';
 import {TemplatePortalDirective} from '../core';
 
+/** Workaround for https://github.com/angular/angular/issues/17849 */
+export const _MdTabLabelBaseClass = TemplatePortalDirective;
+
 /** Used to flag tab labels for use with the portal directive */
 @Directive({
   selector: '[md-tab-label], [mat-tab-label], [mdTabLabel], [matTabLabel]',
 })
-export class MdTabLabel extends TemplatePortalDirective {
+export class MdTabLabel extends _MdTabLabelBaseClass {
   constructor(templateRef: TemplateRef<any>, viewContainerRef: ViewContainerRef) {
     super(templateRef, viewContainerRef);
   }