fix(common): introduce isObservable method (#14067)

Closes #8848

PR Close #14067

modules/@angular/core/src/util/lang.ts

@@ -6,6 +6,9 @@
  * found in the LICENSE file at https://angular.io/license
  */
 
+import {Observable} from 'rxjs/Observable';
+import {$$observable as symbolObservable} from 'rxjs/symbol/observable';
+
 /**
  * Determine if the argument is shaped like a Promise
  */
@@ -14,3 +17,10 @@ export function isPromise(obj: any): obj is Promise<any> {
   // It's up to the caller to ensure that obj.then conforms to the spec
   return !!obj && typeof obj.then === 'function';
 }
+
+/**
+ * Determine if the argument is an Observable
+ */
+export function isObservable(obj: any | Observable<any>): obj is Observable<any> {
+  return !!(obj && obj[symbolObservable]);
+}