Created projects Microsoft.Data.Entity.Migrations.Design and Microsoft.Data.Entity.Migrations.Design.Tests. Moved CSharpMigrationCodeGenerator, CSharpModelCodeGenerator, MigrationScaffolder and related utilities to the .Design project and the corresponding tests to .Design.Tests.
Also increased the CommandTimeout in TestDatabase.cs to 30 seconds (default) to workaround the timeout expired exceptions I get on my machine when running those tests.