Strange Errors when building game

Hi guys. Well I have probuilder and some other tools installed on my project and all was working fine until I went to build the game. It was building fine earlier but now there are errors. I've seen these before and it looks like unity is having problems locating a core library but I can't fix it. Help??

 Assets/AssetStore/ProCore/ProBuilder/API Examples/Icosphere FFT/Scripts/IcoBumpin.cs(72,10): error CS0246: The type or namespace name `AnimationCurve' could not be found. Are you missing an assembly reference?
 Assets/Dummy.cs(5,22): error CS0246: The type or namespace name `MonoBehaviour' could not be found. Are you missing an assembly reference?
 Error building Player because scripts had compiler errors
 Build completed with a result of 'Failed'
 UnityEditor.BuildPlayerWindow:BuildPlayerAndRun()
 UnityEditor.BuildPlayerWindow+BuildMethodException: 27 errors
   at UnityEditor.BuildPlayerWindow+DefaultBuildMethods.BuildPlayer (BuildPlayerOptions options) [0x00207] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:172 
   at UnityEditor.BuildPlayerWindow.CallBuildMethods (Boolean askForBuildLocation, BuildOptions defaultBuildOptions) [0x00050] in C:\buildslave\unity\build\Editor\Mono\BuildPlayerWindowBuildMethods.cs:83 
 UnityEditor.BuildPlayerWindow:BuildPlayerAndRun()

There are more errors but I got all the juicy bits. Strangely my own added script (dummy.cs) is also causing errors.... but it should not be. I also reimported everything but still nothing. Hoping I don't have to copy my assets to another project. I mean how could unity struggle to find monobehaviour??
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Update 1: I've imported everything to a new project and still the error persists. Maybe re-installing unity? I'll keep you posted.
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Update 2:
Well that was really really weird. So I reinstalled unity and then I reimported my project to be sure and it's all working now. Maybe something got deleted or moved within my unity distro....
I hope this was slightly entertaining to read as I was high as a kite through it all.... :)