message: feat: Require use of flag to send envelopes
  This gives us an easy kill-switch if we decide to revert back to the
  store endpoint. Sending envelopes becomes opt-in.
  Usage:
  Sentry.init({
  dsn: '...',
  integrations: [new Sentry.Integrations.Tracing()],
  tracesSampleRate: 1,
  _experiments: { useEnvelope: true },
  });
diff: +import { getCurrentHub } from '@sentry/hub';
  import { Event } from '@sentry/types';
  import { API } from './api';
  @@ -15,7 +16,9 @@ interface SentryRequest {
  /** Creates a SentryRequest from an event. */
  export function eventToSentryRequest(event: Event, api: API): SentryRequest {
  - const useEnvelope = event.type === 'transaction';
  + const client = getCurrentHub().getClient();
  + const experimentsOptions = (client && client.getOptions()._experiments) || {};
  + const useEnvelope = event.type === 'transaction' && experimentsOptions.useEnvelope;
  const req: SentryRequest = {
  body: JSON.stringify(event),
  