Re: Check Engine Light - Knows very little about cars
What mattmag said.
Assuming that the OP's VIN is before the production break, it also appears that the dealer is correct on the ECM replace/reflash. There is a TSB to address a P0420 + P0136 issue. If they also need the converter, I will assume that they have done a test to check the performance of the catalyst. -- The talented hawk speaks French.
I wouldnt be so fast to do anything. I would go to autozone and have it cleared again. If the gas did get in the carbon canister which is common on toyotas it takes some time to go away. I could see the vehicle throw a catylist code with that. Most toyota owners know its a big no no to overfill a toyota.