fix(angular): module exports

angular/src/directives/navigation/ion-back-button.ts

@@ -0,0 +1,19 @@
+import { Directive, HostListener, Optional } from '@angular/core';
+import { IonRouterOutlet } from './ion-router-outlet';
+
+@Directive({
+  selector: 'ion-back-button'
+})
+export class IonBackButton {
+
+  constructor(
+    @Optional() private routerOutlet: IonRouterOutlet,
+  ) {}
+
+  @HostListener('click')
+  onClick() {
+    if (this.routerOutlet.canGoBack()) {
+      this.routerOutlet.pop();
+    }
+  }
+}