That's it. Spec has difference about this.

http://www.w3.org/TR/WebIDL/#named-constructors:
 
A named constructor MUST have a property named “length” with attributes { [[Writable]]: false, [[Enumerable]]: false, [[Configurable]]: false } whose value is a Number determined as follows:

    Initialize S to the effective overload set for constructors with identifier id on interface I and with argument count 0.
    Return the maximum argument list length of the constructors in the entries of S.

@pjckcs, let's reflect the editor's draft.