title: Make asset pipeline act like production on development
body: <p>I am experiencing some problems with assets on <em>production</em>: missing ones, stuff compiled into the wrong files (javascript for "/admin" getting compiled into the frontend code and so on). Most of the assets come from engines. I want to debug and optimize this.</p>
  
  <p>For that, I need to precompile, serve and fail on my <em>development environment</em> just like it is done on <em>production</em>. </p>
  
  <p>I have added some lines to my <code>config/development.rb</code>:</p>
  
  <pre><code>  config.serve_static_assets = true
    config.assets.precompile += %w( store/all.js store/all.css admin/all.js admin/all.css ) # @TODO: clean up, and optimize.
    config.assets.compile = false
  </code></pre>
  
  <p>Running this with <code>rake RAILS_GROUPS=assets RAILS_ENV=development assets:precompile</code> gives me all the assets and the <code>manifest.yml</code> in <code>public/</code>. </p>
  
  <p>But then the server fails: </p>
  
  <pre><code>Sprockets::Helpers::RailsHelper::AssetPaths::AssetNotPrecompiledError in Spree/home#index
  
  Showing /xxxx/app/views/spree/shared/_head.html.erb where line #13 raised:
  
  favicon.ico isn't precompiled
  </code></pre>
  
  <p><strong>favicon.ico isn't precompiled</strong>. But it is! Its there, in the public dir, in <code>manifest.yml</code>, and I can fetch it with the browser (or wget): <a href="http://localhost:3000/assets/favicon.ico" rel="nofollow">http://localhost:3000/assets/favicon.ico</a>.</p>
  
  <p><strong>NOTE</strong> Favicon is simply the first asset called. If I strip out favicon, the problem simply surfaces with the next asset, being "all.js", or, when that is stripped, "all.css", and so on. I can strip it untill "footer_bg.png", and it will then fail there. Again: the problem is not favicon, but the fact that the <em>development</em> environment does not see the precompiled assets at all.</p>
  
  <p>What more is needed to get <em>development</em> asset pipeline similar to <em>production</em>?</p>
  
  <p>EDIT: More explicit explanation that favicon is not the problem, merely a symptom.</p>