feat(Rx.Symbol.rxSubscriber): add rxSubscriber symbol

spec/symbol/rxSubscriber-spec.js

@@ -0,0 +1,13 @@
+var Rx = require('../../dist/cjs/Rx');
+var RxKitchenSink = require('../../dist/cjs/Rx.KitchenSink');
+var Symbol = require('../../dist/cjs/util/SymbolShim').SymbolShim;
+
+describe('rxSubscriber symbol', function () {
+  it('should exist on Rx', function () {
+    expect(Rx.Symbol.rxSubscriber).toBe(Symbol.for('rxSubscriber'));
+  });
+
+  it('should exist on Rx.KitchenSink', function () {
+    expect(RxKitchenSink.Symbol.rxSubscriber).toBe(Symbol.for('rxSubscriber'));
+  });
+});

src/Rx.KitchenSink.ts

@@ -146,13 +146,18 @@ import {ImmediateScheduler} from './scheduler/ImmediateScheduler';
 import {TimeInterval} from './operator/extended/timeInterval';
 import {TestScheduler} from './testing/TestScheduler';
 import {VirtualTimeScheduler} from './scheduler/VirtualTimeScheduler';
+import {rxSubscriber} from './symbol/rxSubscriber';
 /* tslint:enable:no-unused-variable */
 
 /* tslint:disable:no-var-keyword */
 var Scheduler = {
   nextTick,
   immediate
 };
+
+var Symbol = {
+  rxSubscriber
+};
 /* tslint:enable:no-var-keyword */
 
 export {
@@ -171,5 +176,6 @@ export {
     ObjectUnsubscribedError,
     TestScheduler,
     VirtualTimeScheduler,
-    TimeInterval
+    TimeInterval,
+    Symbol
 };

src/Rx.ts

@@ -122,13 +122,18 @@ import {immediate} from './scheduler/immediate';
 import {nextTick} from './scheduler/nextTick';
 import {NextTickScheduler} from './scheduler/NextTickScheduler';
 import {ImmediateScheduler} from './scheduler/ImmediateScheduler';
+import {rxSubscriber} from './symbol/rxSubscriber';
 /* tslint:enable:no-unused-variable */
 
 /* tslint:disable:no-var-keyword */
 var Scheduler = {
   nextTick,
   immediate
 };
+
+var Symbol = {
+  rxSubscriber
+};
 /* tslint:enable:no-var-keyword */
 
 export {
@@ -137,6 +142,7 @@ export {
     Observable,
     Subscriber,
     Subscription,
+    Symbol,
     AsyncSubject,
     ReplaySubject,
     BehaviorSubject,

src/symbol/rxSubscriber.ts

@@ -0,0 +1,9 @@
+import {SymbolShim} from '../util/SymbolShim';
+
+/**
+ * rxSubscriber symbol is a symbol for retreiving an "Rx safe" Observer from an object
+ * "Rx safety" can be defined as an object that has all of the traits of an Rx Subscriber,
+ * including the ability to add and remove subscriptions to the subscription chain and
+ * guarantees involving event triggering (can't "next" after unsubscription, etc).
+ */
+export const rxSubscriber = SymbolShim.for('rxSubscriber');