You can still install it as a local extension in developer mode:
1. Go to <chrome://extensions> and turn on developer mode (top right)
-2. Download <a href="../plain/build/newtab.crx" download>build/newtab.crx</a> (make sure you save it, don't install it straight away - see below)
+2. Download <a href="../plain/build/newtab.crx" download>build/newtab.crx</a> ("Save link as...", don't install it straight away - see below)
3. Drag the file onto the extensions page to install it
Opening the crx file using Chrome's default file handler causes a "CRX_REQUIRED_PROOF_MISSING" error because Chrome now only allows extensions obtained from their store. However, you can drag and drop it onto the extensions page and that seems to bypass the verification. More info [here](https://support.google.com/chrome/thread/3125155?hl=en).