fix(decorators): fix InstanceCheck decorator

src/@ionic-native/core/decorators.ts

@@ -119,7 +119,7 @@ export function InstanceCheck(opts: CordovaCheckOptions = {}) {
   return (pluginObj: Object, methodName: string, descriptor: TypedPropertyDescriptor<any>): TypedPropertyDescriptor<any> => {
     return {
       value: function(...args: any[]): any {
-        if (instanceAvailability(pluginObj)) {
+        if (instanceAvailability(this)) {
           return descriptor.value.apply(this, args);
         } else {
 

src/@ionic-native/plugins/google-maps/index.ts

@@ -111,10 +111,7 @@ export class GoogleMap {
    */
   @InstanceCheck({ observable: true })
   on(eventName: string): Observable<any> {
-    return new Observable((observer) => {
-        this._objectInstance.on(eventName, observer.next.bind(observer));
-        return () => this._objectInstance.off(event);
-    });
+    return Observable.fromEvent(this._objectInstance, eventName);
   }
 
   /**