I was surprised when I tried to spread a `Set` in TypeScript and the browser complained about `Set.slice()` not being defined. This is because TypeScript does not automatically enable support for iterators when targeting earlier ES versions, like we do in the browser, unless you use the `"downlevelIteration": true` compiler option. This injects some helpers into the necessary files for reading/spreading iterators, which can be stuffed behind an import statement with using the `"importHelpers": true` compiler option and include `tslib` in our dependencies. This is already a dependency of several of our packages, so it shouldn't cause any additional modules.
This PR upgrades RxJS to version 6 and switches to a fork of `stream-to-observable` which includes an updated version of `any-observable` that supports RxJS 6 until https://github.com/jamestalmage/stream-to-observable/pull/10 is merged. The primary change in this version of RxJS is the movement of stream operators from `Obersable.prototype` to the `rxjs/operators` module. Some of the operators, like `catch` and `do`, have been renamed (`catchError`, and `tap`). The Obsevable factories have also been moved from static methods on the `Observable` class to named exports of the root `rxjs` module. Some of those factories have also changed slightly, like `fromEvent` which now emits arrays if the event handler is called with multiple arguments.
```js
// import the Rx namespace to get the Observable factories
import * as Rx from 'rxjs';
// import the operators as named imports
import { map, tap, switchMap } from 'rxjs/operators';
```
* [kbn-dev-utils/procRunner] try using execa to avoid never-exitting procs
* [kbn-dev-utils/procRunner] don't listen for close event, rely on stdio streams ending
