firebase serve is serving http1.1 not http2

### [REQUIRED] Environment info

**firebase-tools:** 8.9.0

**Platform:** macOS high sierra version 10.13.6

### [REQUIRED] Test case

```bash
firebase serve
```

### [REQUIRED] Steps to reproduce

Run `firebase init` to create a basic project using default settings and then run `firebase serve`. All resources are served as http1.1, however firebase hosting is supposed to use http2.

### [REQUIRED] Expected behavior

The local development server which is started by `firebase serve` uses http2.

### [REQUIRED] Actual behavior

The local development server which is started by `firebase serve` uses http1.1.

Start by running `firebase init` with default settings and then `firebase serve` on the reported firebase-tools 8.9.0 environment, checking the protocol used by the local server. Trace the `firebase serve` entry point into the local hosting server; done means local resources are served over HTTP/2 rather than HTTP/1.1.

Written by the indexing model from the issue text.