The use of Date makes things look odd, but I assume the spec means
something like the following:*
js> function f(){}
js> f.prototype = {const prop1: 1, prop2: 2}
js> o = new f
js> o.prop1 = 42
42
js> o.prop1
1
which is the behaviour I'd expect.
* I can't find the notation for const's in object literals.