Created Unassigned: Make DbSetClause publicly constructible [1409]

Workitem #1136 - "Make update command trees publicly constructible" is closed but should not be in my opinion.

The constructor for DbUpdateCommandTree takes a parameter ReadOnlyCollection<DbModificationClause> setClauses. DbModificationClause has an internal constructor so it is not possible to pass anything other than null or a pre-existing collection of set clauses.

Why make DbUpdateCommandTree publicly constructible and then none of the other expression classes?