arrow functions should not be allowed as constructors

[erwinmombay]

https://closure-compiler-debugger.appspot.com/#input0%3D%252F**%250A%2520%2520*%2520%2540param%2520%257Bfunction(new%253AObject)%257D%2520newable%250A%2520%2520*%252F%250Afunction%2520register(newable)%2520%257B%250A%2520%2520return%2520new%2520newable()%253B%250A%257D%250A%250Aclass%2520A%2520%257B%257D%250A%250Aregister(A)%253B%250Aregister(()%2520%253D%253E%2520%257B%257D)%253B%26input1%26conformanceConfig%26externs%26refasterjs-template%26CHECK_TYPES%3Dtrue%26REWRITE_MODULES_BEFORE_TYPECHECKING%3Dtrue%26CLOSURE_PASS%3Dtrue%26PRESERVE_TYPE_ANNOTATIONS%3Dtrue%26PRETTY_PRINT%3Dtrue

see example above. we have the parameter @param {function(new:Object)} newable and we pass in a () => arrow function which will be new'd up and we get Uncaught TypeError: newable is not a constructor

[shicks]

This is tricky because we don't currently store whether a function is an arrow or not anywhere in the type model. And because newing classical functions is so common, it's not clear what the rules should be for it. I'm not sure what would happen if we said that a non-constructor is not a subtype of a constructor - likely the world would break.