To reproduce:
1. Open attached project
Observe the compile errors:
Assets/Test.cs(6,34): error CS0012: The type `System.Configuration.ConfigurationSectionGroup' is defined in an assembly that is not referenced. Consider adding a reference to assembly `System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

Assets/Test.cs(7,34): error CS0012: The type `System.Configuration.ConfigurationSection' is defined in an assembly that is not referenced. Consider adding a reference to assembly `System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

Assets/Test.cs(8,34): error CS0012: The type `System.Configuration.ConfigurationElement' is defined in an assembly that is not referenced. Consider adding a reference to assembly `System.Configuration, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b03f5f7f11d50a3a'

These classes are not referenced in Test.cs. The classes actually referenced are, respectively:
System.Net.Configuration.MailSettingsSectionGroup, System.Net.Configuration.SmtpSection, System.Net.Configuration.SmtpNetworkElement

Reproduced on: 5.5.0b6
Not reproduced on: 5.4.1p1