Yep, missing from the docs and the type definitions but it's there and it works!
(context.webAPI as any).execute(request).then(successCallback, errorCallback);
execute Client API docs for more info
(context.webAPI as any) .executeMultiple(requests) .then(successCallback, errorCallback);
executeMultiple Client API docs for more info
Some of the tips 'n' tricks in the PCF Tips 'n' Tricks category are to be used with caution. Although they may work at the time of writing, they may or may not be officially supported by Microsoft