fix(symbol): revert unique symbol in #5874 (#6224)

* fix(symbol): revert unique symbol in #5874

- closes #5919

* fix(types): move Symbol.observable into types.ts (#6178)

Closes #6175

Co-authored-by: Nicholas Jamieson <[email protected]>

Author: kwonoj

api_guard/dist/types/index.d.ts

@@ -240,7 +240,7 @@ 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/types.ts

@@ -5,18 +5,12 @@ 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
+ * 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: unique symbol;
+    readonly observable: symbol;
   }
 }