Consistently use Promise<void> instead of Promise<undefined>

lib.dom.d.ts ubiquitously uses Promise<void> for promises not resolving
to undefined. It has done this for a long time.

When the generator was written, there were 0 or 1 (I can't quite tell)
places where this type of promise occured outside of a function return
value. However, Web Streams are now widely supported, and they have
properties that are promises that resolve to no value. These are
currently typed as Promise<undefined>. This commit changes them to be
typed as Promise<void>, to align with the rest of lib.dom.d.ts.

There are 5 readonly properties that are impacted by this change.

baselines/audioworklet.generated.d.ts

@@ -832,7 +832,7 @@ declare var ReadableStreamDefaultReader: {
 
 interface ReadableStreamGenericReader {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/cancel) */
     cancel(reason?: any): Promise<void>;
 }
@@ -1121,11 +1121,11 @@ declare var WritableStreamDefaultController: {
  */
 interface WritableStreamDefaultWriter<W = any> {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/desiredSize) */
     readonly desiredSize: number | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/abort) */
     abort(reason?: any): Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/close) */

baselines/dom.generated.d.ts

@@ -18564,7 +18564,7 @@ declare var ReadableStreamDefaultReader: {
 
 interface ReadableStreamGenericReader {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/cancel) */
     cancel(reason?: any): Promise<void>;
 }
@@ -25448,7 +25448,7 @@ interface WebTransport {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/incomingUnidirectionalStreams) */
     readonly incomingUnidirectionalStreams: ReadableStream;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/close) */
     close(closeInfo?: WebTransportCloseInfo): void;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/createBidirectionalStream) */
@@ -26055,11 +26055,11 @@ declare var WritableStreamDefaultController: {
  */
 interface WritableStreamDefaultWriter<W = any> {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/desiredSize) */
     readonly desiredSize: number | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/abort) */
     abort(reason?: any): Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/close) */

baselines/serviceworker.generated.d.ts

@@ -194,7 +194,7 @@ interface ExtendableMessageEventInit extends ExtendableEventInit {
 
 interface FetchEventInit extends ExtendableEventInit {
     clientId?: string;
-    handled?: Promise<undefined>;
+    handled?: Promise<void>;
     preloadResponse?: Promise<any>;
     replacesClientId?: string;
     request: Request;
@@ -2475,7 +2475,7 @@ interface FetchEvent extends ExtendableEvent {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/clientId) */
     readonly clientId: string;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/handled) */
-    readonly handled: Promise<undefined>;
+    readonly handled: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/preloadResponse) */
     readonly preloadResponse: Promise<any>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/request) */
@@ -4576,7 +4576,7 @@ declare var ReadableStreamDefaultReader: {
 
 interface ReadableStreamGenericReader {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/cancel) */
     cancel(reason?: any): Promise<void>;
 }
@@ -7793,7 +7793,7 @@ interface WebTransport {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/incomingUnidirectionalStreams) */
     readonly incomingUnidirectionalStreams: ReadableStream;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/close) */
     close(closeInfo?: WebTransportCloseInfo): void;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/createBidirectionalStream) */
@@ -8087,11 +8087,11 @@ declare var WritableStreamDefaultController: {
  */
 interface WritableStreamDefaultWriter<W = any> {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/desiredSize) */
     readonly desiredSize: number | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/abort) */
     abort(reason?: any): Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/close) */

baselines/sharedworker.generated.d.ts

@@ -4410,7 +4410,7 @@ declare var ReadableStreamDefaultReader: {
 
 interface ReadableStreamGenericReader {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/cancel) */
     cancel(reason?: any): Promise<void>;
 }
@@ -7608,7 +7608,7 @@ interface WebTransport {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/incomingUnidirectionalStreams) */
     readonly incomingUnidirectionalStreams: ReadableStream;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/close) */
     close(closeInfo?: WebTransportCloseInfo): void;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/createBidirectionalStream) */
@@ -7920,11 +7920,11 @@ declare var WritableStreamDefaultController: {
  */
 interface WritableStreamDefaultWriter<W = any> {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/desiredSize) */
     readonly desiredSize: number | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/abort) */
     abort(reason?: any): Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/close) */

baselines/webworker.generated.d.ts

@@ -209,7 +209,7 @@ interface ExtendableMessageEventInit extends ExtendableEventInit {
 
 interface FetchEventInit extends ExtendableEventInit {
     clientId?: string;
-    handled?: Promise<undefined>;
+    handled?: Promise<void>;
     preloadResponse?: Promise<any>;
     replacesClientId?: string;
     request: Request;
@@ -2675,7 +2675,7 @@ interface FetchEvent extends ExtendableEvent {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/clientId) */
     readonly clientId: string;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/handled) */
-    readonly handled: Promise<undefined>;
+    readonly handled: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/preloadResponse) */
     readonly preloadResponse: Promise<any>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/FetchEvent/request) */
@@ -4851,7 +4851,7 @@ declare var ReadableStreamDefaultReader: {
 
 interface ReadableStreamGenericReader {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/ReadableStreamBYOBReader/cancel) */
     cancel(reason?: any): Promise<void>;
 }
@@ -8232,7 +8232,7 @@ interface WebTransport {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/incomingUnidirectionalStreams) */
     readonly incomingUnidirectionalStreams: ReadableStream;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/close) */
     close(closeInfo?: WebTransportCloseInfo): void;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WebTransport/createBidirectionalStream) */
@@ -8565,11 +8565,11 @@ declare var WritableStreamDefaultController: {
  */
 interface WritableStreamDefaultWriter<W = any> {
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/closed) */
-    readonly closed: Promise<undefined>;
+    readonly closed: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/desiredSize) */
     readonly desiredSize: number | null;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/ready) */
-    readonly ready: Promise<undefined>;
+    readonly ready: Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/abort) */
     abort(reason?: any): Promise<void>;
     /** [MDN Reference](https://developer.mozilla.org/docs/Web/API/WritableStreamDefaultWriter/close) */

src/build/emitter.ts

@@ -347,7 +347,10 @@ export function emitWebIdl(
     }
     if (!obj.type)
       throw new Error("Missing 'type' field in " + JSON.stringify(obj));
-    const type = convertDomTypeToTsTypeWorker(obj);
+    let type = convertDomTypeToTsTypeWorker(obj);
+    if (type === "Promise<undefined>") {
+      type = "Promise<void>";
+    }
     return obj.nullable ? makeNullable(type) : type;
   }
 
@@ -356,9 +359,6 @@ export function emitWebIdl(
     if (type === "undefined") {
       return "void";
     }
-    if (type === "Promise<undefined>") {
-      return "Promise<void>";
-    }
     if (type === "undefined | PromiseLike<undefined>") {
       return "void | PromiseLike<void>";
     }