message: Replace "undefined" with "void" in return type
diff: @@ -289,6 +289,17 @@ export function emitWebIdl(webidl: Browser.WebIdl, flavor: Flavor, iterator: boo
  return type.nullable ? makeNullable(type.name) : type.name;
  }
  + function convertDomTypeToTsReturnType(obj: Browser.Typed): string {
  + const type = convertDomTypeToTsType(obj);
  + if (type === "undefined") {
  + return "void";
  + }
  + if (type === "Promise<undefined>") {
  + return "Promise<void>";
  + }
  + return type;
  + }
  +
  function convertDomTypeToTsTypeWorker(obj: Browser.Typed): { name: string; nullable: boolean } {
  let type;
  if (typeof obj.type === "string") {
  @@ -542,7 +553,7 @@ export function emitWebIdl(webidl: Browser.WebIdl, flavor: Flavor, iterator: boo
  const m = methods[0];
  const overload = m.signature[0];
  const paramsString = overload.param ? paramsToString(overload.param) : "";
  - const returnType = overload.type ? convertDomTypeToTsType(overload) : "void";
  + const returnType = overload.type ? convertDomTypeToTsReturnType(overload) : "void";
  printer.printLine(`type ${i.name} = ((${paramsString}) => ${returnType}) | { ${m.name}(${paramsString}): ${returnType}; };`);
  }
  printer.printLine("");
  @@ -690,7 +701,7 @@ export function emitWebIdl(webidl: Browser.WebIdl, flavor: Flavor, iterator: boo
  function emitSignature(s: Browser.Signature, prefix: string | undefined, name: string | undefined, printLine: (s: string) => void) {
  const paramsString = s.param ? paramsToString(s.param) : "";
  - let returnType = convertDomTypeToTsType(s);
  + let returnType = convertDomTypeToTsReturnType(s);
  returnType = s.nullable ? makeNullable(returnType) : returnType;
  emitComments(s, printLine);
  printLine(`${prefix || ""}${name || ""}(${paramsString}): ${returnType};`);
  