Erm... There are no rules saying you'd have to use it as an improvised weapon. In in case you didn't read my previous post, using a dart as an improvised weapon would only take away the
finesse property, which seems to be a rather pointless nerf to make. Applying the same logic to nets (because they're the
only other weapon that house rule would affect), then your net would always be used at disadvantage (because of its range of 5/15) unless you used it to make a melee attack, at which point it would become an improvised weapon, deal 1d4 damage, and lose its ability to restrain who it hits. So using it in melee would make your net effectively identical to a chair leg or a dead goblin, and lose all benefits of being a net.
Does that honestly seem like that makes
any sense from a narrative standpoint? Perhaps I haven't impressed upon you enough that ranged
weapons and ranged
attacks aren't mutually inclusive? I'm tiring of repeating myself but let's try again.
When you make a ranged attack with a weapon, spell, or some other means [including but not limited to ranged attacks with ranged weapons, thrown melee weapons, thrown improvised weapons, and spell attacks], you have disadvantage on the attack roll if you are within 5 feet of a hostile creature who can see you and who isn't incapacitated.
Player's Handbook, page 195
So if you're to make a ranged attack, in this case by throwing a dart at an adjacent enemy, that attack is made with disadvantage. However, if the dart never leaves your hand there's no reasonable interpretation that would let you treat that attack as a ranged attack; instead, it would be treated as a melee attack with a ranged weapon. The rules dictating melee attacks are as follows:
Used in hand-to-hand combat, a melee attack allows you to attack a foe within your reach. A melee attack typically uses a handheld weapon such as a sword, a warhammer, or an axe.
Player's Handbook, page 195
Provided that the dart never leaves your hand when you try to stab your foe with it, that attack could certainly be construed as a melee attack, couldn't it? After all, you have a weapon in your hand (satisfying the condition for a "handheld weapon") and you're attacking a target within 5 feet of you. So this attack is treated as a melee attack rather than a ranged attack.
Addressing your comment that it would be used as an improvised weapon, the only relevant rule states:
If you use a weapon that has the ammunition property to make a melee attack, you treat the weapon as an improvised weapon.
Since darts lack the
ammunition property, that rule doesn't apply; when used to make melee attacks, darts retain their
finesse.
There's no label for "ranged weapons which can be used without disadvantage in melee", but that's because it's unnecessary; disadvantage is a result of the type of attack being made, not the type of weapon used.
QED.