react devtools need to be able to connect to the eval-frame

[bcolloran]

related to #1719 -- we need to be able to connect to the eval-frame with react devtools to monitor perf. we might try this: https://github.com/eddyerburgh/react-dev-tools-iframe-webpack-plugin
or even adapt the code directly: https://github.com/eddyerburgh/react-dev-tools-iframe-webpack-plugin/blob/master/index.js

it looks like this code might overwrite the variable that allows react devtools to connect to the main window, so it might be the case that if you connect to the eval frame you cannot connect to the main window. in that case, we would want to put this behind a command line flag

[bcolloran]

(note: this is much less important now that we have so few react components in the eval-frame)