System.Object is not defined or imported when building PCL project in Xamarin Studio 4.0.12

There're also a bunch of similar errors:-

: error CS0518: The predefined type `System.Object' is not defined or imported
: error CS0518: The predefined type `System.ValueType' is not defined or imported
: error CS0518: The predefined type `System.Attribute' is not defined or imported
: error CS0518: The predefined type `System.Int32' is not defined or imported
: error CS0518: The predefined type `System.UInt32' is not defined or imported
. . .

To fix this, go to References -> Edit References -> .Net Assembly then add mscorlib.dll from /Library/Frameworks/Mono.framework/Libraries/mono/4.5

Refs
https://bugzilla.xamarin.com/show_bug.cgi?id=12845