You are looking for:
Microsoft.TeamFoundation.Build.WebApi.BuildDefinition
in
Microsoft.TeamFoundation.Build2.WebApi, Version=14.0.0.0
You can contact them through:
Microsoft.TeamFoundation.Build.WebApi.BuildHttpClient
The easiest way to add it to your project is the Nuget Package:
PM> Install-Package Microsoft.TeamFoundationServer.ExtendedClient
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