[js][database] Add ref().isEqual()

lib/modules/database/reference.js

@@ -337,6 +337,17 @@ export default class Reference extends ReferenceBase {
     return this.path;
   }
 
+  /**
+   * Returns whether another Reference represent the same location and are from the
+   * same instance of firebase.app.App - multiple firebase apps not currently supported.
+   * @param {Reference} otherRef - Other reference to compare to this one
+   * @return {Boolean} Whether otherReference is equal to this one
+   * {@link https://firebase.google.com/docs/reference/js/firebase.database.Reference#isEqual}
+   */
+  isEqual(otherRef: Reference): boolean {
+    return !!otherRef && otherRef.constructor === Reference && otherRef.key === this.key;
+  }
+
   /**
    * GETTERS
    */