fix(plugins): fix rxjs 6 build errors

#2439

src/@ionic-native/plugins/firebase/index.ts

@@ -1,6 +1,6 @@
 import { Injectable } from '@angular/core';
 import { Cordova, IonicNativePlugin, Plugin } from '@ionic-native/core';
-import { Observable } from 'rxjs/Observable';
+import { Observable } from 'rxjs';
 
 /**
  * @beta
@@ -30,7 +30,7 @@ import { Observable } from 'rxjs/Observable';
   plugin: 'cordova-plugin-firebase',
   pluginRef: 'FirebasePlugin',
   repo: 'https://github.com/arnesson/cordova-plugin-firebase',
-  platforms: ['Android', 'iOS'],
+  platforms: ['Android', 'iOS']
 })
 @Injectable()
 export class Firebase extends IonicNativePlugin {
@@ -293,7 +293,10 @@ export class Firebase extends IonicNativePlugin {
     successIndex: 2,
     errorIndex: 3
   })
-  verifyPhoneNumber(phoneNumber: string, timeoutDuration: number): Promise<any> {
+  verifyPhoneNumber(
+    phoneNumber: string,
+    timeoutDuration: number
+  ): Promise<any> {
     return;
   }