The UnityEditor.iOS.Xcode namespace contains useful classes to work with XCode projects and to automate certain Build post-processing steps. However, the code is part of the iOS Build Support and needs to be installed separately. It can therefore not be relied upon to exist, otherwise, Users who don't have the iOS Build Support installed will get a compilation Error. This makes it difficult to use UnityEditor.iOS.Xcode in Editor Tools that are intended to be used across a wide variety of Unity projects. One might try to conditionally compile the code with UNITY_IOS but that has a serious downside: UNITY_IOS is only defined when iOS is the active platform. However, it is possible to do an iOS build without iOS being the active platform using the BuildPipeline API. In this case, the post-processing code using UnityEditor.iOS.Xcode will _not_ be called. The user has to remember to switch platforms before doing a build and that might not be possible when the build is automated.

To reproduce:

1. Download attached “XCodeTest.zip" project and open in Unity
2. Make sure that Active Platform is not iOS
3. Press File > Build iOS

Expected Result: After Build completes, console Prints message:

"Running PBXProject post-processing...
UnityEngine.Debug:Log(Object)
XcodeTest:PostProcessBuild(BuildTarget, String) (at Assets/Editor/XcodeTest.cs:19)
UnityEditor.BuildPipeline:BuildPlayer(EditorBuildSettingsScene[], String, BuildTarget, BuildOptions)
XcodeTest:IosBuild() (at Assets/Editor/XcodeTest.cs:13)"

Actual Result: After Build completes, console Prints message:

"Not running post-processing, UNITY_IOS is not defined...
UnityEngine.Debug:Log(Object)
XcodeTest:PostProcessBuild(BuildTarget, String) (at Assets/Editor/XcodeTest.cs:22)
UnityEditor.BuildPipeline:BuildPlayer(EditorBuildSettingsScene[], String, BuildTarget, BuildOptions)
XcodeTest:IosBuild() (at Assets/Editor/XcodeTest.cs:13)"

Notes:
- I noticed that using BuildPipeline.BuildPlayer this way will change the active platform but not recompile the Editor code. After doing the steps above, iOS is selected as the Active Platform but doing another Build will still _not_ have UNITY_IOS set. Only switching to another Platform and back or making a change that will recompile the Editor Code will set UNITY_IOS
- There are three ways this can be resolved:
1. Unity recompiles the editor code before doing a build to make sure UNITY_IOS is always defined for editor code when doing an iOS build
2. Unity defines scripting define symbol when the iOS Build Support is installed
3. The UnityEditor.iOS.Xcode DLL is bundled with the base Unity installation so that it is always available

Reproduced on Unity 2017.4.0f1, 2017.4.29f1, 2018.3.14f1, 2018.4.2f1, 2019.1.7f1, 2019.2.0b6 and 2019.3.0a6