Fix race condition that's causing preview/console and code to be out of sync (#990)

[ZiyaoWei]

This change attempts to fix #990. The cause seems to be a race condition - similar to #790 - where the state stored in redux is slightly behind the editor. This PR pulls the current file from the editor and use its content instead of the state in redux to compute the preview and console output.

I have verified that this pull request:

• has no linting errors (npm run lint)
• is from a uniquely-named feature branch and has been rebased on top of the latest master. (If I was asked to make more changes, I have made sure to rebase onto master then too)
• is descriptively named and links to an issue number, i.e. Fixes #123

[ZiyaoWei]

Not sure if this is an okay fix, and any feedback are welcomed!

[catarak]

thanks for this! i think this is the right approach.

[catarak]

i'm setting an error in the console:

webpack-internal:///99:20 Warning: Failed prop type: The prop `cmController` is marked as required in `PreviewFrame`, but its value is `undefined`.
    in PreviewFrame (created by IDEView)
    in IDEView (created by Connect(IDEView))
    in Connect(IDEView) (created by withRouter(Connect(IDEView)))
    in withRouter(Connect(IDEView)) (created by RouterContext)
    in div (created by App)
    in App (created by Connect(App))
    in Connect(App) (created by RouterContext)
    in RouterContext (created by Router)
    in Router (created by App)
    in Provider (created by App)
    in App (created by HotExportedApp)
    in AppContainer (created by HotExportedApp)
    in HotExportedApp

I think this is happening because there's a moment when the CodeMirror editor hasn't been instantiated yet. What I did in Nav.jsx (which also has a cmController prop) was make a default value for cmController that's an empty object.

[ZiyaoWei]

Good catch, thanks! Hopefully this latest commit fixed that.

[catarak]

cool, this is working great for me. merging!