Warn about duplicate dependencies

Created by: darrenscerri

Add duplicate-package-checker-webpack-plugin as proposed here: #1844 . Display compilation warnings on build.

This feature can be quickly tested by creating an app, installing an old version of fbjs (yarn add fbjs@0.6.0) and importing fbjs from App.js. This will result in having duplicate fbjs packages since React already requires the most recent version of fbjs.

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Created by: darrenscerri

Does anyone have an idea why build is failing on AppVeyor? It says curl: command not found, but I can see that previous builds with curl have been passing.

Created by: Timer

@gaearon should we put this in a 0.9 release or 0.10?

Created by: Timer

I'm going to punt this to 0.10 for now since some people rely on CI to check warnings and consider them build failures. I don't want to accidentally upset anyone.

Sorry for the wait, @darrenscerri!

Created by: darrenscerri

Ok, makes sense.

Created by: Timer

@darrenscerri could you please close this PR and open a new one from a feature branch that's on master?

Created by: darrenscerri

@Timer Ok done: #2011

Created by: Timer

Thanks so much, @darrenscerri!