The REST API Reference for VS Team Services and TFS page separates the \"Build 1.0\" and \"Build 2.0\" APIs.
It seems like such a difference also exists in the .NET API
You're looking for:
Microsoft.TeamFoundation.Build.WebApi.BuildDefinition
in:
Microsoft.TeamFoundation.Build2.WebApi, Version=14.0.0.0
Which can be accessed through:
Microsoft.TeamFoundation.Build.WebApi.BuildHttpClient
Simplest way to add it to your project is using the Nuget Package:
PM> Install-Package Microsoft.TeamFoundationServer.ExtendedClient