Funnily enough, works by RAW.

There is no prohibition of magical healing in construct traits, though some constructs are explicitly immune to magic except select few effects.

There is also no prohibition in the heal spell.

Technically, you can do it, but expect a heavy book thrown by your DM. He will also probably houserule it away, at least I personally would.

Note:

Can't remember anything on that in 3.5 either, though the description of Warforged contrasts the possibility of both special and positive-energy healing (emphasis mine):

As living constructs, warforged can be affected by spells that target living creatures as well as by those that target constructs. Damage dealt to a warforged can be healed by a cure light wounds spell or a repair light damage spell, for example, and a warforged is vulnerable to disable construct and harm. However, spells from the healing subschool and supernatural abilities that cure hit point damage or ability damage provide only half their normal effect to a warforged.

That leads one to the implication of impossibility of positive-energy based healing for a regular construct, even if it is not stated anywhere.

(dumb) 3.5 RAW:

Negative energy (such as an inflict spell) can heal undead creatures.

That's the only particularity of a undead regarding healing (besides the fact it can't heal itself).

Let's say you target a zombie with Heal. The spell's description tells us:

If used against an undead creature, heal instead acts like harm.

So we have to read the Harm text rules:

Harm charges a subject with negative energy that deals 10 points of damage per caster level (to a maximum of 150 points at 15th level). If the creature successfully saves, harm deals half this amount, but it cannot reduce the target’s hit points to less than 1.

If used on an undead creature, harm acts like heal.

It seems obvious we have to ignore the last line if we want to keep a little sense here. Basically what we learn is that casting heal makes us charge the zombie with negative energy that deals damage. The undead trait says undead can be healed with negative energy, not that every negative energy heals then, so it "makes sense" that you can harm it, with the heal spell, doing negative energy damage.

By the way, note that by casting harm on the zombie, you are supposed to make him gain HP through positive energy. That is the effect of the spell and does not trigger undead special ability.

Pathfinder rules on this are copy-pasted from 3.5, with the same nonsense.

RACP (rules as commonly played):

When you target a undead with positive energy, you make it take damage, when you target it with negative energy, you make it gain HP. That's pretty simple and the weird cases have to be houseruled by the GM.

I can't provide absolute evidence for the fact everyone plays with these rules, but this kind of material for example suggests undeads taking damage from positive energy.