To reproduce:
1. Create a new project
2. Switch to WP8 platform
3. Build...
4. ERROR: Error building Player: Exception: Failed to run reference rewriter with command "--target="Temp/StagingArea\UnityEditor.UI.dll" --framework="C:\Program Files (x86)\Reference Assemblies\Microsoft\Framework\WindowsPhone\v8.0,C:\Program Files (x86)\Microsoft SDKs\Windows Phone\v8.0\Libraries,C:/Program Files/Unity 5.0.0a8/Editor/Data/PlaybackEngines/wp8support/Players/ARM/master" --platform="C:\Program Files (x86)\Windows Phone Kits\8.0\Windows MetaData\Windows.winmd" --support="Temp/StagingArea/WinRTLegacy.dll" --winmdrefs="" --system=System --dbg=pdb --alt=System.Net;System.Net.Sockets,System.Net;System.Xml.Serialization;System.ComponentModel,System.Windows;System.Threading,mscorlib". [Temp/StagingArea\UnityEditor.UI.dll]
Catastrophic failure while running rrw: Mono.Cecil.AssemblyResolutionException: Failed to resolve assembly: 'UnityEditor, Version=0.0.0.0, Culture=neutral, PublicKeyToken=null'
at Unity.ReferenceRewriter.RewriteContext.RewriteResolver.Resolve(AssemblyNameReference name)
at Mono.Cecil.MetadataResolver.Resolve(TypeReference type)
at Unity.ReferenceRewriter.RewriteTypeReferences.Visit(TypeReference type, String referencingEntityName)
at Unity.ReferenceRewriter.ReferenceDispatcher.Dispatch(TypeDefinition type)
at Unity.ReferenceRewriter.ReferenceDispatcher.Dispatch()
at Unity.ReferenceRewriter.RewriteOperation.Execute(RewriteContext context)
at Unity.ReferenceRewriter.Program.Main(String[] args)

Build folder is empty. This issue occurs on both windows 8 and windows 8.1.

There is a workaround - closing and opening the same project with unity solves the problems and allows to build. This is still a pretty major issue, even with the workaround.