Add space before `/>` in self-closing tag

This matches how Prettier formats self-closing JSX tags and makes
expressions such as `<Widget expanded />` more readable.

dev-server/ui-playground/util/jsx-to-string.js

@@ -85,7 +85,7 @@ export function jsxToString(vnode) {
       return `<${name}${propStr}>\n${childrenStr}\n</${name}>`;
     } else {
       // No children - use a self-closing tag.
-      return `<${name}${propStr}/>`;
+      return `<${name}${propStr} />`;
     }
   } else {
     return '';