public void Dispatches_visitor()
{
var dropForeignKeyOperation = new DropForeignKeyOperation("dbo.MyTable", "MyFK");
var mockVisitor = new Mock<MigrationOperationSqlGenerator>(new RelationalTypeMapper());
var builder = new Mock<IndentedStringBuilder>();
dropForeignKeyOperation.GenerateSql(mockVisitor.Object, builder.Object);
mockVisitor.Verify(g => g.Generate(dropForeignKeyOperation, builder.Object), Times.Once());
}