Leakage from new foundation and slab into house

I'm putting an addition on the west side of my stucco slab house. A contractor sawed the stucco along the entire west side to expose the foundation, then poured the new foundation and slab up against the old. Rain was threatening, so he covered the new slab and tucked the plastic up into the sawn out area to keep water out.
The rain came, and my wife happened to be up at 3am to get a glass of water and noticed that the carpet on the west side was wet, so I am out at 3am in the rain trying to figure out what the problem is and I decided all I could do at that point was staple the plastic better to the studs.
The contractor came over the next day, appoligized profusely, and helped me lift the carpet and discard the pad. I put four fans to work drying the carpet. He stapled the plastic more securely to the studs.
Rain came today, and guess what? Water running in the house again. What do folks think should have been done before the rains and what do you think should be done now?
My own thought is that the first thing the contractor should have done upon exposing the sill plate would have been to caulk the crack between the sill plate and the foundation to form a last line of defense against water seeping in. Also, since I think that the water washing down the stucco wall is tending to make a right turn at the 90 degree cut and splash down inside the plastic inside the wall, I would have cut the stucco on an angle so that the water would have to drip straight down and surface tension couldn't pull it around the corner towards the inside of the wall.
Any advice or insight gratefully accepted!
-Harry
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