[core] add extendApp support

lib/firebase-app.js

@@ -1,6 +1,7 @@
 import { NativeModules } from 'react-native';
 
 import INTERNALS from './internals';
+import { isObject } from './utils';
 
 import AdMob, { statics as AdMobStatics } from './modules/admob';
 import Auth, { statics as AuthStatics } from './modules/auth';
@@ -34,6 +35,7 @@ export default class FirebaseApp {
     this.messaging = this._staticsOrModuleInstance(MessagingStatics, Messaging);
     this.perf = this._staticsOrModuleInstance({}, Performance);
     this.storage = this._staticsOrModuleInstance(StorageStatics, Storage);
+    this._extendedProps = {};
   }
 
   /**
@@ -78,6 +80,26 @@ export default class FirebaseApp {
     return Object.assign({}, this._options);
   }
 
+  /**
+   *
+   * @param props
+   */
+  extendApp(props: Object) {
+    if (!isObject(props)) throw new Error(INTERNALS.ERROR_MISSING_ARG('Object', 'extendApp'));
+    const keys = Object.keys(props);
+
+    for (let i = 0, len = keys.length; i < len; i++) {
+      const key = keys[i];
+
+      if (!this._extendedProps[key] && Object.hasOwnProperty.call(this, key)) {
+        throw new Error(INTERNALS.ERROR_PROTECTED_PROP(key));
+      }
+
+      this[key] = props[key];
+      this._extendedProps[key] = true;
+    }
+  }
+
   /**
    *
    * @return {Promise}

lib/internals.js

@@ -63,6 +63,20 @@ export default {
       return `Missing required callback for method ${method}().`;
     },
 
+    /**
+     * @return {string}
+     */
+    ERROR_MISSING_ARG(type, method) {
+      return `Missing required argument of type '${type}' for method '${method}()'.`;
+    },
+
+    /**
+     * @return {string}
+     */
+    ERROR_PROTECTED_PROP(name) {
+      return `Property '${name}' is protected and can not be overridden by extendApp.`;
+    },
+
     /**
      * @return {string}
      * @param namespace