docs: function getEventSourceMappingViaCache(functionName, streamName, context) {
    const functionAndStream = `${functionName}|${streamName}`;
  
    let key = eventSourceMappingKeysByFunctionAndStream.get(functionAndStream);
    if (!key) {
      key = {key: functionAndStream};
      eventSourceMappingKeysByFunctionAndStream.set(functionAndStream, key);
    }
  
    const esmPromiseCached = eventSourceMappingPromisesByKey.get(key);
  
    const esmPromise = esmPromiseCached ?
      esmPromiseCached.catch(err => getEventSourceMapping(functionName, streamName, context)) :
      getEventSourceMapping(functionName, streamName, context);
  
    eventSourceMappingPromisesByKey.set(key, esmPromise);
  
    return esmPromise;
  }