Gatsby: [v2] potential bug with gatsby-plugin-jss: need to clear SheetsRegistry

Description

With certain combinations of styles, the SheetsRegistry should be cleared between rendered pages.

Steps to reproduce

This repo has a simplified reproduction and tracks my fix for it:
https://github.com/cpboyd/gatsby-layout-mui-bug

Note: I'm not sure where resetting the SheetsRegistry should occur. I just placed it after the ToString() call because that seemed the easiest place to put it.

Expected result

The pages should render consistently between the Javascript rendering and static rendering.

Actual result

The styles were jumbled.

Environment

System:
OS: macOS High Sierra 10.13.6
CPU: x64 Intel(R) Core(TM) i7-8750H CPU @ 2.20GHz
Shell: 3.2.57 - /bin/bash
Binaries:
Node: 10.9.0 - /usr/local/bin/node
Yarn: 1.9.4 - /usr/local/bin/yarn
npm: 6.4.0 - /usr/local/bin/npm
Browsers:
Chrome: 68.0.3440.106
Safari: 11.1.2
npmPackages:
gatsby: ^2.0.0-rc.0 => 2.0.0-rc.0
gatsby-image: ^2.0.0-rc.0 => 2.0.0-rc.0
gatsby-plugin-google-analytics: ^2.0.0-rc.0 => 2.0.0-rc.0
gatsby-plugin-jss: ^2.0.2-rc.0 => 2.0.2-rc.0
gatsby-plugin-layout: next => 1.0.0-rc.0
gatsby-plugin-manifest: ^2.0.2-rc.0 => 2.0.2-rc.0
gatsby-plugin-offline: ^2.0.0-rc.0 => 2.0.0-rc.0
gatsby-plugin-react-helmet: ^3.0.0-rc.0 => 3.0.0-rc.0
gatsby-plugin-sharp: ^2.0.0-rc.0 => 2.0.0-rc.0
gatsby-plugin-typescript: ^2.0.0-rc.0 => 2.0.0-rc.1
gatsby-plugin-typography: ^2.2.0-rc.0 => 2.2.0-rc.0
gatsby-remark-external-links: ^0.0.4 => 0.0.4
gatsby-remark-images: ^2.0.1-rc.0 => 2.0.1-rc.0
gatsby-remark-responsive-iframe: ^2.0.0-rc.0 => 2.0.0-rc.0
gatsby-source-filesystem: ^2.0.1-rc.0 => 2.0.1-rc.0
gatsby-transformer-json: ^2.1.1-rc.0 => 2.1.1-rc.0
gatsby-transformer-remark: ^2.1.1-rc.0 => 2.1.1-rc.0
gatsby-transformer-sharp: ^2.1.1-rc.0 => 2.1.1-rc.0
npmGlobalPackages:
gatsby-cli: 1.1.58

File contents (if changed)

gatsby-config.js: N/A
package.json: N/A
gatsby-node.js: N/A
gatsby-browser.js: N/A

The original gatsby-ssr.js API started with a new SheetsRegistry each time it rendered the page:

exports.replaceRenderer = (
  { bodyComponent, replaceBodyHTMLString, setHeadComponents },
  { theme = {} }
) => {
  const sheets = new SheetsRegistry()

  const bodyHTML = renderToString(
    <JssProvider registry={sheets}>
      <ThemeProvider theme={theme}>{bodyComponent}</ThemeProvider>
    </JssProvider>
  )

  replaceBodyHTMLString(bodyHTML)
  setHeadComponents([
    <style
      type="text/css"
      id="server-side-jss"
      key="server-side-jss"
      dangerouslySetInnerHTML={{ __html: sheets.toString() }}
    />,
  ])
}

Now the variable is file scoped and seems to persist across renderings:

import React from "react"
import { JssProvider, SheetsRegistry, ThemeProvider } from "react-jss"

const sheets = new SheetsRegistry()

// eslint-disable-next-line react/prop-types,react/display-name
exports.wrapRootElement = ({ element }, { theme = {} }) => (
  <JssProvider registry={sheets}>
    <ThemeProvider theme={theme}>{element}</ThemeProvider>
  </JssProvider>
)

exports.onRenderBody = ({ setHeadComponents }) => {
  setHeadComponents([
    <style
      type="text/css"
      id="server-side-jss"
      key="server-side-jss"
      dangerouslySetInnerHTML={{ __html: sheets.toString() }}
    />,
  ])