powershell core currently doesn't validate CDXML schema due to the missing of related APIs in .NET Core 1.1 and prior versions, and the related code are if/def'ed out.

In .NET Core 2.0, XML Schema validation APIs are brought back, so the CDXML schema validation related code in ScriptWriter.cs can be enabled now, but it will require additional work to make the schema validation work in powershell core, because

1. The XML Schema file cmdlets-over-objects.xsd is missing in Github
2. The resource string CmdletizationCoreResources.Xml_cmdletsOverObjectsXsd likely needs to be reworked to make the schema validation work.