refactor(Symbol): deprecate $$observable and replace it with observable

To migrate the code follow the example bellow:

Before:

import { $$observable } from 'rxjs/symbol/observable';

After:

import { observable } from 'rxjs/symbol/observable';

Closes ReactiveX#2415

src/Observable.ts

@@ -6,7 +6,7 @@ import { root } from './util/root';
 import { toSubscriber } from './util/toSubscriber';
 import { IfObservable } from './observable/IfObservable';
 import { ErrorObservable } from './observable/ErrorObservable';
-import { $$observable } from './symbol/observable';
+import { observable as Symbol_observable } from './symbol/observable';
 
 export interface Subscribable<T> {
   subscribe(observerOrNext?: PartialObserver<T> | ((value: T) => void),
@@ -177,7 +177,7 @@ export class Observable<T> implements Subscribable<T> {
    * @method Symbol.observable
    * @return {Observable} this instance of the observable
    */
-  [$$observable]() {
+  [Symbol_observable]() {
     return this;
   }
 }

src/Rx.ts

@@ -171,7 +171,7 @@ import { QueueScheduler } from './scheduler/QueueScheduler';
 import { AnimationFrameScheduler } from './scheduler/AnimationFrameScheduler';
 import { $$rxSubscriber as rxSubscriber } from './symbol/rxSubscriber';
 import { $$iterator as iterator } from './symbol/iterator';
-import { $$observable as observable } from './symbol/observable';
+import { observable } from './symbol/observable';
 
 /* tslint:enable:no-unused-variable */
 

src/observable/FromObservable.ts

@@ -11,7 +11,7 @@ import { $$iterator } from '../symbol/iterator';
 import { Observable, ObservableInput } from '../Observable';
 import { Subscriber } from '../Subscriber';
 import { ObserveOnSubscriber } from '../operator/observeOn';
-import { $$observable } from '../symbol/observable';
+import { observable as Symbol_observable } from '../symbol/observable';
 
 /**
  * We need this JSDoc comment for affecting ESDoc.
@@ -84,7 +84,7 @@ export class FromObservable<T> extends Observable<T> {
    */
   static create<T>(ish: ObservableInput<T>, scheduler?: IScheduler): Observable<T> {
     if (ish != null) {
-      if (typeof ish[$$observable] === 'function') {
+      if (typeof ish[Symbol_observable] === 'function') {
         if (ish instanceof Observable && !scheduler) {
           return ish;
         }
@@ -107,9 +107,9 @@ export class FromObservable<T> extends Observable<T> {
     const ish = this.ish;
     const scheduler = this.scheduler;
     if (scheduler == null) {
-      return ish[$$observable]().subscribe(subscriber);
+      return ish[Symbol_observable]().subscribe(subscriber);
     } else {
-      return ish[$$observable]().subscribe(new ObserveOnSubscriber(subscriber, scheduler, 0));
+      return ish[Symbol_observable]().subscribe(new ObserveOnSubscriber(subscriber, scheduler, 0));
     }
   }
 }

src/symbol/observable.ts

@@ -18,4 +18,9 @@ export function getSymbolObservable(context: any) {
   return $$observable;
 }
 
-export const $$observable = getSymbolObservable(root);
+export const observable = getSymbolObservable(root);
+
+/**
+ * @deprecated use observable instead
+ */
+export const $$observable = observable;

src/util/subscribeToResult.ts

@@ -8,7 +8,7 @@ import { $$iterator } from '../symbol/iterator';
 import { Subscription } from '../Subscription';
 import { InnerSubscriber } from '../InnerSubscriber';
 import { OuterSubscriber } from '../OuterSubscriber';
-import { $$observable } from '../symbol/observable';
+import { observable as Symbol_observable } from '../symbol/observable';
 
 export function subscribeToResult<T, R>(outerSubscriber: OuterSubscriber<T, R>,
                                         result: any,
@@ -67,8 +67,8 @@ export function subscribeToResult<T>(outerSubscriber: OuterSubscriber<any, any>,
         break;
       }
     } while (true);
-  } else if (result && typeof result[$$observable] === 'function') {
-    const obs = result[$$observable]();
+  } else if (result && typeof result[Symbol_observable] === 'function') {
+    const obs = result[Symbol_observable]();
     if (typeof obs.subscribe !== 'function') {
       destination.error(new TypeError('Provided object does not correctly implement Symbol.observable'));
     } else {