fix(symbol): revert unique symbol in #5874

- closes #5919

api_guard/dist/types/index.d.ts

@@ -238,7 +238,7 @@ export interface ObjectUnsubscribedError extends Error {
 
 export declare const ObjectUnsubscribedError: ObjectUnsubscribedErrorCtor;
 
-export declare const observable: string | SymbolConstructor["observable"];
+export declare const observable: string | symbol;
 
 export declare class Observable<T> implements Subscribable<T> {
     protected operator: Operator<any, T> | undefined;

src/internal/symbol/observable.ts

@@ -1,2 +1,11 @@
+/** Symbol.observable addition */
+/* Note: This will add Symbol.observable globally for all TypeScript users,
+  however, we are no longer polyfilling Symbol.observable */
+declare global {
+  interface SymbolConstructor {
+    readonly observable: symbol;
+  }
+}
+
 /** Symbol.observable or a string "@@observable". Used for interop */
 export const observable = (() => (typeof Symbol === 'function' && Symbol.observable) || '@@observable')();

src/internal/types.ts

@@ -4,22 +4,6 @@
 import { Observable } from './Observable';
 import { Subscription } from './Subscription';
 
-/**
- * NOTE: This will add Symbol.observable globally for all TypeScript users,
- * however, we are no longer polyfilling Symbol.observable. Note that this will be at
- * odds with older version of @types/node and symbol-observable which incorrectly define
- * `Symbol.observable` as `symbol` rather than `unique symbol`. "What about not defining
- * this non-standard symbol at all?" you might ask... Well, that ship has sailed. There are
- * dozens of libraries using this symbol now and many of them are quite popular.
- * So here we are, and it's probably my fault. Sorry, "the web", if I have hurt you,
- * the world just needed a standard way to provide interop for these types. -Ben
- */
-declare global {
-  interface SymbolConstructor {
-    readonly observable: unique symbol;
-  }
-}
-
 /** OPERATOR INTERFACES */
 
 export interface UnaryFunction<T, R> {