$RequiredAssembliesForInternalWebProxy = @( [System.Net.IWebProxy].Assembly.FullName, 
                                            [System.Uri].Assembly.FullName )

$SourceForInternalWebProxy = @" 
using System; 
using System.Net;

namespace Microsoft.PowerShell.Commands.PowerShellGet 
{ 
    /// <summary>
    /// Used by Ping-Endpoint function to supply webproxy to HttpClient
    /// We cannot use System.Net.WebProxy because this is not available on CoreClr
    /// </summary>
    public class InternalWebProxy : IWebProxy
    {
        Uri _proxyUri;
        ICredentials _credentials;

        public InternalWebProxy(Uri uri, ICredentials credentials)
        {
            Credentials = credentials;
            _proxyUri = uri;
        }

        /// <summary>
        /// Credentials used by WebProxy
        /// </summary>
        public ICredentials Credentials
        {
            get
            {
                return _credentials;
            }
            set
            {
                _credentials = value;
            }
        }

        public Uri GetProxy(Uri destination)
        {
            return _proxyUri;
        }

        public bool IsBypassed(Uri host)
        {
            return false;
        }
    }
} 
"@

Add-Type -ReferencedAssemblies $RequiredAssembliesForInternalWebProxy `
         -TypeDefinition $SourceForInternalWebProxy `
         -Language CSharp `
         -ErrorAction SilentlyContinue

Add-Type : Unable to load one or more of the requested types. Retrieve the LoaderExceptions property for more
information.
At line:1 char:1
+ Add-Type -ReferencedAssemblies $RequiredAssembliesForInternalWebProxy ...
+ ~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
    + CategoryInfo          : NotSpecified: (:) [Add-Type], ReflectionTypeLoadException
    + FullyQualifiedErrorId : System.Reflection.ReflectionTypeLoadException,Microsoft.PowerShell.Commands.AddTypeCommand

PS D:\>
PS D:\> $Error[0].Exception.LoaderExceptions
Method 'GetProxy' in type 'Microsoft.PowerShell.Commands.PowerShellGet.InternalWebProxy' from assembly 'nxg304ea.wnz,
Version=0.0.0.0, Culture=neutral, PublicKeyToken=null' does not have an implementation.

> $PSVersionTable

Name                           Value
----                           -----
PSCompatibleVersions           {1.0, 2.0, 3.0, 4.0...}
PSVersion                      6.0.0-alpha
WSManStackVersion              3.0
PSRemotingProtocolVersion      2.3
GitCommitId                    v6.0.0-alpha.12-104-g67e9bf6376b90c289d725c904280a2439de4dd3b
PSEdition                      Core
BuildVersion                   3.0.0.0
CLRVersion
SerializationVersion           1.1.0.1