add AsyncBuiltinIterator and switch existing AsyncIterableIterator uses to that

Author: bakkot

src/lib/es2018.asyncgenerator.d.ts

@@ -1,6 +1,6 @@
 /// <reference lib="es2018.asynciterable" />
 
-interface AsyncGenerator<T = unknown, TReturn = any, TNext = unknown> extends AsyncIterator<T, TReturn, TNext> {
+interface AsyncGenerator<T = unknown, TReturn = any, TNext = unknown> extends AsyncBuiltinIterator<T, TReturn, TNext> {
     // NOTE: 'next' is defined using a tuple to ensure we report the correct assignability errors in all places.
     next(...args: [] | [TNext]): Promise<IteratorResult<T, TReturn>>;
     return(value: TReturn | PromiseLike<TReturn>): Promise<IteratorResult<T, TReturn>>;

src/lib/es2018.asynciterable.d.ts

@@ -16,6 +16,10 @@ interface AsyncIterator<T, TReturn = any, TNext = undefined> {
     throw?(e?: any): Promise<IteratorResult<T, TReturn>>;
 }
 
+interface AsyncBuiltinIterator<T, TReturn = void, TNext = undefined> extends AsyncIterator<T, TReturn, TNext> {
+    [Symbol.asyncIterator](): AsyncBuiltinIterator<T, TReturn, TNext>;
+}
+
 interface AsyncIterable<T> {
     [Symbol.asyncIterator](): AsyncIterator<T>;
 }