title: Unity 5.1.1 error : dll is not allowed to be included or could not be found
body: <p>I'm new to unity and upgrading one of my app using Unity 5.1.1. It was working fine with 4.6.2, but I'm getting following error after upgradation</p>
  
  <pre><code>ArgumentException: The Assembly UnityEditor is referenced by iGUI ('Assets/lib/igui/Runtime/iGUI.dll'). But the dll is not allowed to be included or could not be found.
  UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache, BuildTarget target) (at /Users/builduser/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:154)
  UnityEditor.AssemblyHelper.AddReferencedAssembliesRecurse (System.String assemblyPath, System.Collections.Generic.List`1 alreadyFoundAssemblies, System.String[] allAssemblyPaths, System.String[] foldersToSearch, System.Collections.Generic.Dictionary`2 cache, BuildTarget target) (at /Users/builduser/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:160)
  UnityEditor.AssemblyHelper.FindAssembliesReferencedBy (System.String[] paths, System.String[] foldersToSearch, BuildTarget target) (at /Users/builduser/buildslave/unity/build/Editor/Mono/AssemblyHelper.cs:192)
  UnityEditor.HostView:OnGUI()
  </code></pre>
  
  <p>I tried change compatibility from ".net 2.0 subset" to ".net 2.0", but that didn't worked.</p>
  
  <p>Any help will be appreciated</p>
  
  <p>Thanks in advance</p>