Update:  So after tweeting about this I had a massive fanboy moment when Dan Abramov replied, got the details and then fixed the problem in v2.0.3. So to fix this you just need to run:

yarn add react-scripts@2.0.3
or
npm install react-scripts@2.0.3

No longer a problem

Just did an upgrade of create-react-app react-scripts using

yarn add react-scripts@next

Then discovered after a yarn build that there was no runtime javascript being created

In the asset-manifest.json the files below that aren't being generated are
/react/static/js/runtime~main.bf2515a3.js
/react/static/js/runtime~main.bf2515a3.js.map

The files that are generated are

./css/main.ebab72c8.chunk.css
./css/1.ae8ee9e3.chunk.css
./js/main.e6406d5b.chunk.js
./js/1.0f883466.chunk.js.map
./js/1.0f883466.chunk.js
./js/main.e6406d5b.chunk.js.map

?
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{
  <span class="hljs-attr">"main.css"</span>: <span class="hljs-string">"/react/static/css/main.ebab72c8.chunk.css"</span>,
  <span class="hljs-attr">"main.js"</span>: <span class="hljs-string">"/react/static/js/main.e6406d5b.chunk.js"</span>,
  <span class="hljs-attr">"main.js.map"</span>: <span class="hljs-string">"/react/static/js/main.e6406d5b.chunk.js.map"</span>,
  <span class="hljs-attr">"static/css/1.ae8ee9e3.chunk.css"</span>: <span class="hljs-string">"/react/static/css/1.ae8ee9e3.chunk.css"</span>,
  <span class="hljs-attr">"static/js/1.0f883466.chunk.js"</span>: <span class="hljs-string">"/react/static/js/1.0f883466.chunk.js"</span>,
  <span class="hljs-attr">"static/js/1.0f883466.chunk.js.map"</span>: <span class="hljs-string">"/react/static/js/1.0f883466.chunk.js.map"</span>,
  <span class="hljs-attr">"runtime~main.js"</span>: <span class="hljs-string">"/react/static/js/runtime~main.bf2515a3.js"</span>,
  <span class="hljs-attr">"runtime~main.js.map"</span>: <span class="hljs-string">"/react/static/js/runtime~main.bf2515a3.js.map"</span>,
  <span class="hljs-attr">"index.html"</span>: <span class="hljs-string">"/react/index.html"</span>
}

When the runtime~main chunk doesn't get created you don't get a react page

The fix is to change the file webpack.config.prod.js file in the <create-react-app-project>/node_modules/react-scripts/config/webpack.config.prod.js

You need to change the runtimeChunk setting to false

After this change yarn build creates the files

I think some of the downside to this quick fix is the react-scripts will be overwritten when you make changes using yarn. So probably you should do a yarn eject and then edit the configuration files when eject copies them into your project giving you full control.