To reproduce:
1. Open the attached project. It contains the FSharp.Core.dll
2. Build for WP8, WP8.1, Win 8.1
3. Error is thrown:

Error building Player: Exception: Failed to run Serialization weaver with cmdline "Temp/StagingArea\Plugins\WSA\FSharp.Core.dll" -pdb -verbose -unity-engine="Temp/StagingArea\Managed/UnityEngine.dll" "Temp/StagingArea\TempSerializationWeaver" .[Temp/StagingArea\Plugins\WSA\FSharp.Core.dll]
Symbols will be read from Temp/StagingArea\Managed/UnityEngine.pdb
Weaving assembly C:\Users\Vilius\Downloads\case_674666-BugReportProject\Temp\StagingArea\Plugins\WSA\FSharp.Core.dll
No symbols for Temp/StagingArea\Plugins\WSA\FSharp.Core.dll
+ Microsoft.FSharp.Core.Unit
+ Microsoft.FSharp.Core.SealedAttribute
+ Microsoft.FSharp.Core.AbstractClassAttribute
+ Microsoft.FSharp.Core.EqualityConditionalOnAttribute
+ Microsoft.FSharp.Core.ComparisonConditionalOnAttribute
+ Microsoft.FSharp.Core.AllowNullLiteralAttribute
+ Microsoft.FSharp.Core.VolatileFieldAttribute
+ Microsoft.FSharp.Core.DefaultAugmentationAttribute
+ Microsoft.FSharp.Core.CLIEventAttribute
+ Microsoft.FSharp.Core.CLIMutableAttribute
+ Microsoft.FSharp.Core.AutoSerializableAttribute
+ Microsoft.FSharp.Core.DefaultValueAttribute
+ Microsoft.FSharp.Core.EntryPointAttribute
+ Microsoft.FSharp.Core.ReferenceEqualityAttribute
+ Microsoft.FSharp.Core.StructuralComparisonAttribute
+ Microsoft.FSharp.Core.StructuralEqualityAttribute
+ Microsoft.FSharp.Core.NoEqualityAttribute
+ Microsoft.FSharp.Core.CustomEqualityAttribute
+ Microsoft.FSharp.Core.CustomComparisonAttribute
+ Microsoft.FSharp.Core.NoComparisonAttribute
+ Microsoft.FSharp.Core.ReflectedDefinitionAttribute
+ Microsoft.FSharp.Core.CompiledNameAttribute
+ Microsoft.FSharp.Core.StructAttribute
+ Microsoft.FSharp.Core.MeasureAttribute
+ Microsoft.FSharp.Core.MeasureAnnotatedAbbreviationAttribute
+ Microsoft.FSharp.Core.InterfaceAttribute
+ Microsoft.FSharp.Core.ClassAttribute
+ Microsoft.FSharp.Core.LiteralAttribute
+ Microsoft.FSharp.Core.FSharpInterfaceDataVersionAttribute
+ Microsoft.FSharp.Core.CompilationMappingAttribute
+ Microsoft.FSharp.Core.CompilationSourceNameAttribute
+ Microsoft.FSharp.Core.CompilationRepresentationAttribute
+ Microsoft.FSharp.Core.ExperimentalAttribute
+ Microsoft.FSharp.Core.CompilationArgumentCountsAttribute
+ Microsoft.FSharp.Core.CustomOperationAttribute
+ Microsoft.FSharp.Core.ProjectionParameterAttribute
+ Microsoft.FSharp.Core.StructuredFormatDisplayAttribute
+ Microsoft.FSharp.Core.CompilerMessageAttribute
+ Microsoft.FSharp.Core.UnverifiableAttribute
+ Microsoft.FSharp.Core.NoDynamicInvocationAttribute
+ Microsoft.FSharp.Core.OptionalArgumentAttribute
+ Microsoft.FSharp.Core.GeneralizableValueAttribute
+ Microsoft.FSharp.Core.RequiresExplicitTypeArgumentsAttribute
+ Microsoft.FSharp.Core.RequireQualifiedAccessAttribute
+ Microsoft.FSharp.Core.AutoOpenAttribute
+ Microsoft.FSharp.Core.PrivateEnvironment
+ Microsoft.FSharp.Core.MatchFailureException
+ Microsoft.FSharp.Core.FSharpTypeFunc
+ Microsoft.FSharp.Core.FuncConvert
+ Microsoft.FSharp.Collections.MapDebugViewKeyValuePair
+ Microsoft.FSharp.Reflection.UnionCaseInfo
+ Microsoft.FSharp.Reflection.FSharpType
+ Microsoft.FSharp.Reflection.FSharpValue
+ Microsoft.FSharp.Math.BigNat
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Joint/_Unbreakable
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Joint/Breakable
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Joint/Broken
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Joint
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Layout/Leaf
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Layout/Node
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Layout/Attr
+ Microsoft.FSharp.Text.StructuredPrintfImpl.Layout
System.NullReferenceException: Object reference not set to an instance of an object.
at Unity.CecilTools.Extensions.TypeReferenceExtensions.IsEnum(TypeReference type)
at Unity.SerializationLogic.UnitySerializationLogic.IsNonSerialized(TypeReference typeDeclaration)
at Unity.Serialization.Weaver.MethodEmitterBase.EmitMethodBody()
at Unity.Serialization.Weaver.SerializationWeaver.AddSerializeMethod()
at Unity.Serialization.Weaver.SerializationWeaver.Weave()
at usw.Weaver.WeaveAssembly(String assemblyPath, AssemblyDefinition unityEngineAssemblyDefinition)
at usw.Weaver.Weave()
at usw.Program.RunProgram(ConversionOptions options)
at usw.Program.Main(String[] args)

Building windows standalone works fine.