add visual indicator for dev mode

static/index.html

@@ -23,6 +23,11 @@
         console.error(error.stack || error);
       }
     }
+
+    document.addEventListener('DOMContentLoaded', function() {
+      if(window.location.params.devMode == "true")
+        document.body.setAttribute('class', 'is-dev-mode')
+    })
   </script>
 
   <script src='../src/stdlib/require.js'></script>