Async for .NET and Silverlight 4
Microsoft has released the final version of Microsoft.Bcl.Async. This package allows developers using .NET 4, Silverlight 4, and Windows Phone 7.5 to use the async features found in C# 5 and VB 11.
The package consists of three libraries: Microsoft.Bcl, Microsoft.Bcl.Async, and Microsoft.Bcl.Build. Due to the way that assembly unification is used, all projects in the solution need to reference all three libraries
Microsoft.Bcl.Async supersedes the Async Targeting Pack, which has been delisted from NuGet.
Ian Culling, Andy Powell & Lee Cunningham Dec 11, 2013