It always throws a type errors, even if we are not in strict mode. the correct behaviour here according to spec is to be a nop. The spill setters do it right.

    @SuppressWarnings("unused")
    private static void setField(final CallSiteDescriptor desc, final PropertyMap oldMap, final PropertyMap newMap, final MethodHandle setter, final Object self, final Object value) throws Throwable {
        final ScriptObject obj = (ScriptObject)self;
        final boolean isStrict = NashornCallSiteDescriptor.isStrict(desc);
        if (!obj.isExtensible()) {
            throw typeError("object.non.extensible", desc.getNameToken(2), ScriptRuntime.safeToString(obj));
        } else if (obj.compareAndSetMap(oldMap, newMap)) {
            setter.invokeExact(self, value);
        } else {
            obj.set(desc.getNameToken(CallSiteDescriptor.NAME_OPERAND), value, isStrict);
        }
    }

As this is so small, and we haven't gotten in trouble yet, I suggest postponing to 8u20