Features

• Add interface to allow container implementors to expose a more native service lookup for required services (#378)
• Rename AddInstance to AddSingleton (#319)
• Create a Microsoft.Extensions.DependencyInjection.Specification.Tests package (#270)

Bugs Fixed

• Should it be possible to add primitive types? (#382)
• Improve error message for "No parameterless constructor defined for this object" (#310)