Function assert

tools/check_docs_lib.ts:44–52

(
  condition: boolean,
  message: string,
  document: { location: Location },
)

}

function assert(
  condition: boolean,
  message: string,
  document: { location: Location },
): asserts condition {
  if (!condition) {
    diagnostics.push(new DocumentError(message, document));
  }
}

function isVoidOrPromiseVoid(returnType: TsTypeDef) {
  return isVoid(returnType) ||