Facebook.Unity/UnityReferences/5.xml

  <ItemGroup>
    <Reference Include="UnityEngine">
      <HintPath>$(UNITY_ENGINE_DIR)UnityEngine.dll</HintPath>
    </Reference>
    <Reference Include="UnityEngine.UI">
      <HintPath>$(UNITY_UI_DIR)UnityEngine.UI.dll</HintPath>
    </Reference>
  </ItemGroup>