Remove global Observable<T>["@@observable"] method declaration. (#7888)

Should fix #7839.

src/utilities/observables/Observable.ts

@@ -8,15 +8,13 @@ export type ObservableSubscription = ZenObservable.Subscription;
 export type Observer<T> = ZenObservable.Observer<T>;
 export type Subscriber<T> = ZenObservable.Subscriber<T>;
 
-// Use global module augmentation to add RxJS interop functionality. By
-// using this approach (instead of subclassing `Observable` and adding an
-// ['@@observable']() method), we ensure the exported `Observable` retains all
-// existing type declarations from `@types/zen-observable` (which is important
-// for projects like `apollo-link`).
-declare global {
-  interface Observable<T> {
-    ['@@observable'](): Observable<T>;
-  }
+// The zen-observable package defines Observable.prototype[Symbol.observable]
+// when Symbol is supported, but RxJS interop depends on also setting this fake
+// '@@observable' string as a polyfill for Symbol.observable.
+const { prototype } = Observable;
+const fakeObsSymbol = '@@observable' as keyof typeof prototype;
+if (!prototype[fakeObsSymbol]) {
+  prototype[fakeObsSymbol] = function () { return this; };
 }
-(Observable.prototype as any)['@@observable'] = function () { return this; };
+
 export { Observable };