fix(Subscriber): use only local Subscriber instances

This removes any checks for `Subscriber` symbol or . That was a micro-optimization that proved to be problematic for node projects with more than one instance of RxJS.

This means that all local `Subject` instances will be wrapped by
`SafeSubscriber`, but prior to that, they were wrapped in
`SubjectSubscriber` anyhow, so the difference is minimal.

Leaves behind `SubjectSubscriber` and implementations of `rxSubscriber`,
which are still functioning, but now cruft.

spec/Subscriber-spec.ts

@@ -1,7 +1,6 @@
 import { expect } from 'chai';
 import { SafeSubscriber } from 'rxjs/internal/Subscriber';
 import { Subscriber } from 'rxjs';
-import { rxSubscriber } from 'rxjs/internal/symbol/rxSubscriber';
 
 /** @test {Subscriber} */
 describe('Subscriber', () => {
@@ -20,17 +19,6 @@ describe('Subscriber', () => {
     expect(times).to.equal(2);
   });
 
-  it('should accept subscribers as a destination if they meet the proper criteria', () => {
-    const fakeSubscriber = {
-      [rxSubscriber](this: any) { return this; },
-      add() { /* noop */ },
-      syncErrorThrowable: false
-    };
-
-    const subscriber = new Subscriber(fakeSubscriber as any);
-    expect((subscriber as any).destination).to.equal(fakeSubscriber);
-  });
-
   it('should wrap unsafe observers in a safe subscriber', () => {
     const observer = {
       next(x: any) { /* noop */ },

src/internal/Subscriber.ts

@@ -72,13 +72,10 @@ export class Subscriber<T> extends Subscription implements Observer<T> {
           break;
         }
         if (typeof destinationOrNext === 'object') {
-          // HACK(benlesh): For situations where Node has multiple copies of rxjs in
-          // node_modules, we cannot rely on `instanceof` checks
-          if (isTrustedSubscriber(destinationOrNext)) {
-            const trustedSubscriber = destinationOrNext[rxSubscriberSymbol]() as Subscriber<any>;
-            this.syncErrorThrowable = trustedSubscriber.syncErrorThrowable;
-            this.destination = trustedSubscriber;
-            trustedSubscriber.add(this);
+          if (destinationOrNext instanceof Subscriber) {
+            this.syncErrorThrowable = destinationOrNext.syncErrorThrowable;
+            this.destination = destinationOrNext;
+            destinationOrNext.add(this);
           } else {
             this.syncErrorThrowable = true;
             this.destination = new SafeSubscriber<T>(this, <PartialObserver<any>> destinationOrNext);
@@ -308,7 +305,3 @@ export class SafeSubscriber<T> extends Subscriber<T> {
     _parentSubscriber.unsubscribe();
   }
 }
-
-export function isTrustedSubscriber(obj: any) {
-  return obj instanceof Subscriber || ('syncErrorThrowable' in obj && obj[rxSubscriberSymbol]);
-}

src/internal/symbol/rxSubscriber.ts

@@ -1,3 +1,4 @@
+/** @deprecated do not use, this is no longer checked by RxJS internals */
 export const rxSubscriber =
   (typeof Symbol === 'function' && typeof Symbol.for === 'function')
     ? Symbol.for('rxSubscriber')

src/internal/util/canReportError.ts

@@ -1,4 +1,4 @@
-import { isTrustedSubscriber, Subscriber } from '../Subscriber';
+import { Subscriber } from '../Subscriber';
 import { Subject } from '../Subject';
 
 /**
@@ -12,7 +12,7 @@ export function canReportError(observer: Subscriber<any> | Subject<any>): boolea
     const { closed, destination, isStopped } = observer as any;
     if (closed || isStopped) {
       return false;
-    } else if (destination && isTrustedSubscriber(destination)) {
+    } else if (destination && destination instanceof Subscriber) {
       observer = destination;
     } else {
       observer = null;