Microsoft.Scripting.Ast is the original version of the DLR expression trees; for .NET 4, these were added to System.Linq.Expressions. .NET 4 builds should use System.Linq.Expressions (and NOT link to Microsoft.Scripting.Core); .NET 2/3/3.5 builds should
use Microsoft.Scripting.Ast and link to Microsoft.Scripting.Core.
IronPython uses a namespace alias that points to the correct one depending on the CLR version being built:
using MSAst = System.Linq.Expressions;
using MSAst = Microsoft.Scripting.Ast;