fix(core): use presence of .subscribe to detect observables rather then Symbol.observable (#15171)

Fixes #14298
Fixes #14473
Fixes #14926

packages/core/src/util/lang.ts

@@ -7,7 +7,6 @@
  */
 
 import {Observable} from 'rxjs/Observable';
-import {$$observable as symbolObservable} from 'rxjs/symbol/observable';
 
 /**
  * Determine if the argument is shaped like a Promise
@@ -22,7 +21,8 @@ export function isPromise(obj: any): obj is Promise<any> {
  * Determine if the argument is an Observable
  */
 export function isObservable(obj: any | Observable<any>): obj is Observable<any> {
-  return !!(obj && obj[symbolObservable]);
+  // TODO use Symbol.observable when https://github.com/ReactiveX/rxjs/issues/2415 will be resolved
+  return !!obj && typeof obj.subscribe === 'function';
 }
 
 // TODO(misko): replace with Object.assign once we require ES6.