Assert that resolved atom shell module paths exist

spec/module-cache-spec.coffee

@@ -14,6 +14,7 @@ describe 'ModuleCache', ->
 
     for builtinName, builtinPath of builtins
       expect(require.resolve(builtinName)).toBe builtinPath
+      expect(fs.isFileSync(require.resolve(builtinName)))
 
     expect(Module._findPath.callCount).toBe 0