If you did a fresh install of xcode 9 and didn't run the gui yet, you may get errors like
[432/27476] COPY_BUNDLE_DATA gen/cef/cefclient_xibs_compile_ibtool/MainMenu.nib cefclient.app/Contents/Resources/English.lproj/MainMenu.nib
FAILED: cefclient.app/Contents/Resources/English.lproj/MainMenu.nib
and/or spew like
DVTPlugInLoading: Failed to load code for plug-in com.apple.dt.IDE.IDEInterfaceBuilderCocoaTouchIntegration (/Applications/Xcode.app/Contents/PlugIns/IDEInterfaceBuilderCocoaTouchIntegration.framework), error = Error Domain=NSCocoaErrorDomain Code=3587 "dlopen_preflight(/Applications/Xcode.app/Contents/PlugIns/IDEInterfaceBuilderCocoaTouchIntegration.framework/IDEInterfaceBuilderCocoaTouchIntegration): Library not loaded: /Library/Developer/PrivateFrameworks/CoreSimulator.framework/Versions/A/CoreSimulator
Referenced from: /Applications/Xcode.app/Contents/Developer/Library/PrivateFrameworks/SimulatorKit.framework/Versions/A/SimulatorKit
Reason: image not found" UserInfo={NSLocalizedFailureReason=The bundle is damaged or missing necessary resources., NSLocalizedRecoverySuggestion=Try reinstalling the bundle., NSFilePath=/Applications/Xcode.app/Contents/PlugIns
Solution: run the gui once and accept the 'install the rest of the tools?' prompt.
See also https://bugzilla.xamarin.com/show_bug.cgi?id=59709